Being a VoIP engineer, and having a few different VoIP and SIP solutions setup on the 900, including Vonage, and a SIP solution that routes through an online pbx trunk, I'd say that call quality is fairly good. The problem I do have however is with "call volume'. This could be masking some audio QoS issues. You may want to check the WiFi "power setting" on your 900 and see if you're using "100mW" vs "10mW"