Sorry! Yes, it'c CCME.

What do you mean the microphone is muted? Does the mute button lit? Or there is just no voice path ?

What I mean is that when the handset is picked up and the user tries to speak into the microphone I can't hear anything on the other phone (where I'm located) and also he cannot hear me when I try to speak. Everything works fine if the other phone is not an IP Phone, that is if the call is not a local one or if the phone is connected using a ATA 186. It seems like there is no voice path.

Are the phones local in the same VLAN? Are these SCCP phones configured under ephone and ephone-dn ?

Yes they are SCCP phones configured using epone and ephone-dn. There is no VLAN defined because it's a small network only with a few users and it's main and only purpose is to act as a office telephony network.

Please mention exact IOS used and phone FW (from settings menu).

The IOS is IP Voice one with the following version "Cisco IOS Software, 2800 Software (C2800NM-IPVOICEK9-M), Version 12.4(22)T, RELEASE SOFTWARE (fc1)". I cannot give you the phone FW because they are not near me. I can get that information only in the morning (it's midnight here).

You should be able to see that with "show ephone phone-load". I assume the phones are all connected to a switch, get DHCP from the router in a same subnet, etc.

The phone FW reported by the show ephone phone-load is SCCP11.8-3-2S. Yes they are connected to a switch and get the DHCP from router in the same subnet.

Your problem is very strange. I recommend you start upgrading to 12.4(22)T1. If that doesn't help you should look at upgrading phone FW as well.

Ok. I will try that. Tnak you!

P.S. Couldn't be some RTP or some codec related issue?

Evidently RTP is not being received correctly across 7911s. There is no evident reason for this to happen.