Ringback heard during call

I had one user reporting that the people she was on the phone with heard a ringback tone during the call. I replaced her phone and it stopped on her extension. It then started happening on another users extension. I upgraded all phone firmwares to the latest versions and it then stopped and came back on the original users extension. So, I reset her phone and now the problem stopped on hers again and started happening only a third users phone.

The infrastructure is Six grandstream gxp 2130’s -> cisco sg200 switch -> sonicwall tz205. The issue is only happening to one phone at a time and it doesnt look like a grandstream problem. What else can it be? What am I missing here?

I should also mention that ringback is only heard by the other side of the call.

PS everything else as far as phone system goes is perfectly fine.

Thanks for any help you can provide.

Has anyone ever had any ringback issues during a call?

I can’t find an answer anywhere.

What type of trunking are you using? Analog perhaps?

Im using voipms for outbound sip trunking and callcentric for inbound sip trunking.

I just disabled the sip account on the grandstream phone and used x lite with the same extension the ringback is happening on and it worked normally. However, I don’t think it could be a grandstream problem because it’s happening only on 2 out of 5 phones now and when I replace the physical phone the new phone does the ringback and the replaced phone works perfectly in a different location.

I can’t figure out where to look to pinpoint the problem.