"The best business telephones in the world."

My issue appears to be identical to that referenced here.

In summary - if overlap dialling is enabled on the handset (Snom 320 in our case) and the dialled number routes to something that doesn't signal ringing (SIP 180 response), then the phone never completes the call.

By example, consider the following two scenarios:

1. A destination that signals ringing (e.g. another extension) [301].  This works as expected.

2. A destination that immediately answers (e.g. a conference line) [501].  This doesn't work with firmware > v8.7 but works with older (e.g. v8.4.x) firmware.

Scenario 1 looks like this:

Phone: INVITE 3@... 

PBX: SIP/2.0 484 Address Incomplete

Phone: INVITE 30@... 

PBX: SIP/2.0 484 Address Incomplete

Phone: INVITE 301@... 

PBX: SIP/2.0 100 Trying

Phone: ACK

PBX: SIP/2.0 180 Ringing

Phone: ACK

PBX: SIP/2.0 200 OK

Phone: ACK

Call proceeds as expected...

Scenario 2 looks like this:

Phone: INVITE 5@... 

PBX: SIP/2.0 484 Address Incomplete

Phone: INVITE 50@... 

PBX: SIP/2.0 484 Address Incomplete

Phone: INVITE 501@... 

PBX: SIP/2.0 100 Trying

Phone: ACK

PBX: SIP/2.0 200 OK

Phone: ACK

Call never connects (i.e. phone still waits for digits and never establishes the RTP session).

It appears that the handset is requiring a 180 Ringing response from the UAS prior to the 200 response.  Unfortunately, this isn't always going to happen when the destination connects immediately.