See my answer in Speed Up SPA3102 . If you are treating this device as an incoming trunk (using Inbound Routes, etc.), then the relevant Dial Plan should be something like
(<:12125551212>S0)
(replace 12125551212 with the DID number used in your route).
If you still have trouble: If a failing call is logged in the PBX, post the log. If not, post a screenshot of the SPA3102 info page, taken when the call has been ringing for about 10 seconds.