POST /api/v2/conversations/calls 403 Forbidden

trying to kick off an outbound call for an agent via the conversations api. sending a post to /api/v2/conversations/calls with a basic json body but getting a 403 forbidden back.

{
 "to": [{"phoneNumber": "+15550199"}],
 "from": {"phoneNumber": "+15550100"}
}

the token works for reading contacts so auth isn’t totally broken. just need to make the call t. what am i missing?