Mitigating SIP 488 Not Acceptable Here Errors in Genesys Cloud Inbound Trunks by Aligning Offer/Answer SDP Payloads with Carrier Codec Preferences
What This Guide Covers
This guide details the precise configuration and architectural alignment required to eliminate SIP 488 Not Acceptable Here failures on Genesys Cloud inbound trunks. You will configure payload type mappings, SDP attribute parameters, and SIP profile negotiation behavior to ensure Genesys Cloud generates valid RFC 3264 SDP answers that match carrier SDP offers. The end result is a trunk that successfully negotiates media streams without dropping inbound calls during the initial INVITE/200 OK handshake.
Prerequisites, Roles & Licensing
- Licensing Tier: CX 1, CX 2, or CX 3 with Telephony features enabled. No WEM or Speech Analytics add-on is required for trunk negotiation.
- Granular Permissions:
Telephony > Trunk > Edit,Telephony > SIP Profile > Edit,Telephony > Codec > View,Telephony > SIP Trunk > View Diagnostics - OAuth Scopes:
telephony:trunk:write,telephony:codec:read,telephony:sipprofile:edit,telephony:diagnostics:read - External Dependencies: Carrier SDP offer documentation or live SIP trace exports, RTP firewall rules permitting dynamic port ranges (default 10000-20000 UDP), carrier provisioning of trunk IP ranges and authentication credentials.
The Implementation Deep-Dive
1. Capture and Analyze the SDP Offer/Answer Mismatch via SIP Tracing
Genesys Cloud rejects an inbound INVITE with SIP 488 when the SDP offer contains payload types, codec identifiers, or attribute parameters that do not map to the trunk configuration. The platform does not attempt fallback negotiation on unknown payload types. You must isolate the exact mismatch before modifying trunk settings.
Enable SIP diagnostics on the affected trunk or request a full SIP trace from the carrier gateway. Locate the initial INVITE and the subsequent 488 Not Acceptable Here response. Extract the SDP payload from both messages. Compare the m=audio line, a=rtpmap, a=fmtp, and a=ptime attributes.
Example Carrier SDP Offer (INVITE):
m=audio 49152 RTP/AVP 96 97 101 0 8
a=rtpmap:96 G722/8000
a=rtpmap:97 G729/8000
a=fmtp:97 annexb=yes
a=rtpmap:101 opus/48000/2
a=fmtp:101 maxplayback=60000;stereo=0
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=ptime:20
Genesys Cloud Default Behavior:
Genesys Cloud maintains an internal codec registry. When it receives payload type 97 for G.729, it expects standard RFC 3551 mapping. If the carrier assigns G.729 to payload type 96 and G.722 to 97, or if the carrier omits a=fmtp:97 annexb=yes, Genesys Cloud generates a 488 because the SDP answer cannot validate the media parameters.
The Trap: Engineers frequently assume that enabling a codec in the trunk configuration resolves the 488. Enabling a codec only activates it for negotiation. It does not override payload type mapping or relax a=fmtp validation. If the carrier uses non-standard payload type assignments, Genesys Cloud will continue rejecting the INVITE regardless of codec enablement.
Architectural Reasoning: We analyze the raw SDP payload first because Genesys Cloud operates as a strict SDP answer generator. The platform follows RFC 3264 Section 6.1.1, which mandates that an answer must only contain payload types present in the offer. If the offer contains a payload type that does not match Genesys Cloud configuration, the platform rejects the entire transaction rather than attempting partial media establishment. This prevents one-way audio or codec mismatch degradation downstream.
2. Configure Carrier-Specific Payload Type Mappings in Genesys Cloud
Genesys Cloud uses standardized payload types by default (PCMA=8, PCMU=0, G.729=18, G.722=9). Carriers frequently reassign these dynamically to optimize routing tables or comply with legacy gateway configurations. You must explicitly map the carrier payload types to Genesys Cloud internal codec identifiers.
Use the Genesys Cloud Telephony Trunk API to inject payload type mappings directly into the trunk configuration. This bypasses UI limitations and ensures exact attribute alignment.
API Request:
PUT https://{org-domain}.mygenesys.cloud/api/v2/telephony/providers/edges/trunks/{trunkId}
Authorization: Bearer {access_token}
Content-Type: application/json
JSON Payload:
{
"name": "Carrier_Inbound_Trunk_01",
"type": "sip",
"enabled": true,
"codecConfiguration": {
"codecMappings": [
{
"payloadType": 96,
"codecName": "G722",
"sampleRate": 8000,
"channels": 1,
"payloadTypeMapping": "custom"
},
{
"payloadType": 97,
"codecName": "G729",
"sampleRate": 8000,
"channels": 1,
"payloadTypeMapping": "custom"
},
{
"payloadType": 101,
"codecName": "OPUS",
"sampleRate": 48000,
"channels": 2,
"payloadTypeMapping": "custom"
}
],
"enabledCodecs": ["G722", "G729", "OPUS", "PCMA", "PCMU"]
},
"sipProfile": {
"id": "{sip_profile_id}"
}
}
The Trap: Setting payloadTypeMapping to dynamic or omitting the codecMappings array forces Genesys Cloud to rely on RFC defaults. Carriers that assign G.729 to payload type 10 or 97 will trigger immediate 488 rejections because the SDP answer generator cannot locate the expected a=rtpmap entry. Dynamic mapping is only viable when the carrier strictly adheres to RFC 3551 static assignments.
Architectural Reasoning: We enforce explicit payload type mapping because Genesys Cloud trunk negotiation occurs at the SIP edge before traffic reaches the media servers. The edge validates the SDP offer against the configured mapping table. Explicit mapping reduces negotiation latency by eliminating fallback lookups and prevents race conditions during high-concurrency inbound spikes. Under load, implicit mapping causes increased CPU cycles on the SIP proxy, which manifests as delayed 200 OK responses and carrier-side timeout retries.
3. Align SDP Attributes (fmtp, ptime, maxptime) with Carrier Expectations
Payload type mapping alone does not guarantee successful negotiation. Genesys Cloud validates a=fmtp strings and packetization time parameters strictly. If the carrier SDP offer includes a=fmtp:97 annexb=yes and the trunk configuration does not permit the annexb parameter, the platform rejects the offer. Similarly, mismatched ptime values cause Genesys Cloud to generate an SDP answer that the carrier gateway refuses.
Configure codec parameters to match the carrier SDP offer exactly. Use the Codec Configuration API to inject fmtp attributes and ptime defaults.
API Request:
PUT https://{org-domain}.mygenesys.cloud/api/v2/telephony/providers/edges/codec-configurations/{codecConfigId}
Authorization: Bearer {access_token}
Content-Type: application/json
JSON Payload:
{
"codecName": "G729",
"sampleRate": 8000,
"channels": 1,
"payloadType": 97,
"parameters": {
"fmtpAttributes": [
{
"key": "annexb",
"value": "yes"
}
],
"ptime": 20,
"maxptime": 20,
"packetizationMode": 0
},
"priority": 1
}
The Trap: Leaving ptime unset defaults Genesys Cloud to 20ms for G.729 and 30ms for PCMA/PCMU. Carriers that mandate a=ptime:30 on G.729 or a=ptime:20 on wideband codecs will reject the SDP answer. The platform does not interpolate packetization times. It matches exactly or fails with 488.
Architectural Reasoning: We enforce strict ptime and fmtp alignment because packetization time dictates jitter buffer allocation and RTP timestamp generation on the media server. If Genesys Cloud answers with a different ptime than the carrier offer, the media server must transcode or repacketize, which consumes DSP resources and increases latency. Explicit alignment allows the media server to pass RTP streams directly without transcoding, preserving bandwidth and reducing CPU utilization during peak call volumes.
4. Validate SIP Profile Negotiation Settings and SDP Generation Behavior
The SIP profile attached to the inbound trunk controls how Genesys Cloud constructs the SDP answer and handles early media. Misconfigured SIP profile settings cause the platform to omit required attributes or reject offers that contain early media indicators.
Configure the SIP profile to match carrier negotiation behavior. Focus on SDP placement, early media handling, and SIP method validation.
API Request:
PUT https://{org-domain}.mygenesys.cloud/api/v2/telephony/providers/edges/sip-profiles/{profileId}
Authorization: Bearer {access_token}
Content-Type: application/json
JSON Payload:
{
"name": "Carrier_SIP_Profile_Inbound",
"enabled": true,
"sdInInvite": true,
"sdInAck": false,
"earlyMedia": {
"enabled": false,
"supportedMethods": ["INVITE"]
},
"sdpAnswerGeneration": {
"strictFmtpValidation": true,
"allowMissingPtime": false,
"rtpPortRange": {
"start": 10000,
"end": 20000
}
},
"sipOptions": {
"requireSdpInInvite": true,
"rejectUnsupportedPayloadTypes": true
}
}
The Trap: Enabling sdInAck forces Genesys Cloud to defer SDP answer generation until the final ACK. Carriers that expect the SDP answer in the 200 OK response will timeout or return 488 when the platform delays media negotiation. Additionally, enabling earlyMedia without carrier support causes the platform to reject INVITEs that contain a=sendrecv with early media indicators.
Architectural Reasoning: We disable sdInAck and enforce SDP in the 200 OK because the majority of carrier gateways operate on synchronous offer/answer models. Deferring SDP generation increases call setup latency and exposes the trunk to carrier-side retransmission storms. By forcing SDP answer generation in the 200 OK, we ensure the carrier gateway validates media parameters immediately. This reduces SIP transaction state retention on the carrier side and prevents memory leaks during high-volume inbound campaigns.
Validation, Edge Cases & Troubleshooting
Edge Case 1: Carrier SDP Offer Contains Non-Standard Payload Types for Standard Codecs
The failure condition: Inbound calls fail with SIP 488. SIP traces show the carrier assigning PCMA to payload type 10 and PCMU to payload type 11. Genesys Cloud logs indicate SDP answer generation failed: payload type mismatch.
The root cause: Genesys Cloud expects PCMA=8 and PCMU=0 per RFC 3551. The carrier gateway uses legacy payload type assignments to avoid conflicts with other trunk providers sharing the same IP pool. The trunk configuration lacks explicit mapping for these non-standard assignments.
The solution: Inject explicit payload type mappings for PCMA and PCMU into the trunk configuration using the codecMappings array. Set payloadType to 10 for PCMA and 11 for PCMU. Verify the carrier SDP offer includes a=rtpmap:10 PCMA/8000 and a=rtpmap:11 PCMU/8000. Do not enable dynamic payload mapping, as it will revert to RFC defaults during trunk failover.
Edge Case 2: Mismatched Packetization Time (ptime) Values Triggering Strict Negotiation Failure
The failure condition: SIP 488 occurs only during peak traffic windows. Off-peak calls succeed. SIP traces reveal the carrier SDP offer contains a=ptime:30 for G.729, but Genesys Cloud answers with a=ptime:20.
The root cause: The carrier gateway dynamically adjusts ptime based on network congestion algorithms. During peak traffic, the carrier increases packetization time to reduce packet count. Genesys Cloud trunk configuration locks ptime at 20ms. The strict validation setting rejects the answer because the packetization times do not match exactly.
The solution: Configure the codec parameters to accept variable ptime values by setting maxptime to 40 and enabling allowMissingPtime to false. Alternatively, contact the carrier to lock ptime at 20ms for Genesys Cloud trunks. If carrier modification is not feasible, adjust the Genesys Cloud codec configuration to match the carrier peak ptime value and validate that the media server jitter buffer accommodates the increased packet interval without introducing audio gaps.
Edge Case 3: Genesys Cloud SDP Answer Generation Fails When Carrier Omits ‘a=fmtp’ on G.729
The failure condition: SIP 488 responses contain Reason: SDP validation error. Missing required fmtp attribute for payload type 18. The carrier SDP offer includes a=rtpmap:18 G729/8000 but omits a=fmtp:18 annexb=yes.
The root cause: The Genesys Cloud codec configuration enforces annexb=yes for G.729. The carrier gateway strips a=fmtp attributes during SDP normalization to reduce payload size. Genesys Cloud strict validation rejects the offer because the required parameter is absent.
The solution: Modify the codec configuration to make annexb optional. Remove the explicit fmtpAttributes requirement from the G.729 codec profile or set strictFmtpValidation to false in the SIP profile. Validate that the media server handles G.729 without annexb extension data. If annexb is required for compliance, request the carrier to preserve a=fmtp attributes during SDP normalization. Cross-reference this configuration with WEM recording settings to ensure speech analytics transcription pipelines do not fail on missing fmtp metadata.