Symptom: Incoming calls ring on the phone but user cannot 'answer' the line, it just keeps ringing. Then spits this error: VOICE_IEC-3-GW: SIP: Internal Error (ACK wait timeout): IEC=220.127.116.11.67.0 on callID 80063 GUID=CC0DCEB1C5A411E8B7C2DD4A89C36D36
Setup: 2921 Primary router/circuit - FXO Card with analog line connected as inbound. HSRP is setup to fallback to secondary router with a secondary circuit and tunnel back to CallManager site. We've tried an 819, 891 and a 2901 router as the secondary with the same results, however when we use a 4331 router calls process just fine.
We have more than one site having the same issue, all our sites with 4331 routers do not have the issue. Are we running into a hardware limitation of some kind or do the 2900 series routers require a different configuration? I read about HA CUBE configuration but that requires two like for like routers which we don't have at some of our sites.