I decided to take a look the new Cisco Small Business Pro IP Phones 500 Series and to make a couple of tests. In my case the phones are SPA502G and SPA508G. I connected them behind UC520, with SIP Trunk connection with the provider. Configured dial-peer for the SIP trunk and set g711alaw as codec that I want to use. When I dial someone outside from one of these phones the voice quality is awful.I tried to change the codecs and so on. The only solution that I could found is in ephone configuration to set codec g729r8 dspfarm-assistant to make transcoding. After that when I made the call the quality for the voice is very good. These phones use spa5x5-7-4-3.bin firmware version.
I did a comparison and made a call from 7941 IP Phone without any additional configuration, and believe me the quality of voice is crystal.
Any tips? Is it possible to observe this problem because of firmware? Is there a new version?
Like I suspected, the SIP trunk seems to be doing G711alaw. The SPA500 phone should be able to negotiate it without issue, so maybe we have a bug. Can you ask your provider to do ulaw instead? I think this will help, but it is just a guess for now.
On my side, I will do some research to see if we are aware of issues with ALAW.