When calling a Siemens Gigaset A580ip from a Siemens Gigaset S685ip both on open vfx. Called phone rings. Calling phone gets engaged tone. If Called phone is picked up call works. If A580 calls S685 same issue.

Siemens manual says
Busy tone: No codec match between the calling and called party.

Trace below is S685 calling A580, and hanging up before answer.
 


Remember if the called phone picks up, the call works fine.