POST /api/v2/conversations/calls returns 400 - malformed participant address

Trying to kick off an outbound call via the API. I’m sending a POST to /api/v2/conversations/calls with a simple payload, but it keeps failing with a 400 Bad Request. The error says Malformed participant address. I’ve double-checked the to address and it’s a valid E.164 number. Here’s the JSON I’m sending:

{
 "to": "+442071234567",
 "from": "+442079876543"
}

Any idea what’s wrong?