SIP - RFC3261
OnSIP uses the SIP protocol as defined in RFC 3261.
DTMF is supported as defined in RFC 2833.
REFER requests as defined in RFC 3515 are currently only supported on originated calls. That is, from the customer perspective, an inbound call can be bridged to an outbound call, but an outbound call cannot be bridged to another outbound call.
Comfort Noise (CN) as defined in RFC 3389 is not fully supported. Please disable Voice Activity Detection (VAD) on your system.