We have a Trixbox 2.8 box running Asterisk 1.6 and sometimes when a call comes in, the person taking the call will hear a continues faint ringing in the background through the duration of the call. We are using a SIP trunk through bandwidth.com and have sent packet captures to them. They say that it's nothing on their end and are pointing the finger at Asterisk. The problem only appears to happen for a few of the employees, and it is completely random. The employees are using Polycom IP 550 telephones.
We've updated components by running yum update, re-wired ethernet cables so that the phones go directly into the switch, replaced the switch, replaced the router, removed the "r" option from the inbound trunk options, and replaced the ring group with an IVR. None of these have fixed the issue. From what I've seen, no one has heard of anything like this. Does anyone have any ideas on what I can try next?