I ordered a couple dozen SPA942s for testing purposes. I provisioned half dozen of them and connected to our server then started going down the checklist of features we support. Make a call… check… receive a call… check… speaker to handset… check… handset to speaker… check… blind transfer… check… attended transfer… check… three way call… wtf? … three way call… fail.
Here are the symptoms…
1 - Answer a call on the SPA942 (aka Caller A)
2 - Press CONF softkey (this puts Caller A on hold and gives them music)
3 - Dial the number you want to include in the three way call (aka: Caller B)
4 - Call connects… awesome…
5 - Click CONF softkey
Step 5 fails. If you press the CONF softkey then you can barely hear caller B and Caller A continues to receive hold music. If you press the JOIN sofkey then it connects Caller A and Caller B but the SPA942 is disconnected from the call.
This problem is caused by using g729 as the preferred codec. If you change to g711 then the problem is corrected. The root cause appears to be the failure of the SPA942 to send an invite to take the initial call back off hold to get the 3 way conference connected.
I hope this helps save someone else an afternoon of frustration.