We're currently testing an external (tie) trunk that is connected to a third-party SIP endpoint where some calls gets transferred back to an external number through that external trunk. Below is a detailed scenario:
- Call from the PSTN arrives at an Edge via an external trunk (and via an SBC).
- Call is transferred to a second external trunk on the same Edge with the 'Take Back and Transfer' feature enabled on the external trunk.
- The SIP endpoint on the second external trunk performs a same-session SIP REFER request to transfer the call to another PSTN number.
- The Edge 'takes back' the call from the second external trunk and transfers the call to the PSTN number requested.
- The outbound PSTN call eventually answers the call. However there is no call progress tone heard by the caller - just silence until the call is answered.
The issue here is that the lack of call progress tones (i.e. usually ring tone from the PSTN) is a really bad caller experience. The caller could be hearing silence indefinitely if the called party does not answer the call.
In contrast, the Transfer to Number block in an Inbound Call Flow in PureCloud presents the caller with call progress tones and is also able to determine whether or not the transfer failed (e.g. busy, rejected etc).
The Take Back and Transfer feature should behave like the Transfer To Number block does with call progress tones - or at least present the caller with locally generated ring tone when a 180 Ringing or 183 Session Progress is received for the outbound call.
Question is, is this a bug or was it just an expected behaviour for the Take Back and Transfer trunk feature as currently implemented?