I have been having a consistent problem with my office phones for about a year now. The provider has pointed the problem to the router. I have replaced my cisco small business router with an enterprise isr 2911 router, and still the issue persists.
The problem happens weekly . The user will only experience one way audio and can not reach out if trying to place a call. What I have to do to resolve the problem is log into the device (phone) and reset the sip port form its original port to another sip port.
So if it is on 5060 , I change it to 5040 and problem resolves. The provider states it is a problem with my router/firewall, but can not tell me what or how to fix.
Router - Cisco ISR 2911
Phones- Grandstream GXP 2100
Please help !