I have my google voice number forwarding to my sipgate account on my n900. I use thias when I am connected to wifi at home because I get bad reception.

The quality when I am using it is not very good at all. If both of us talk at the same time, the sound cuts out for both of us, making it difficult to hold a conversation. I don't notice these issues at all when my phone number is forwarded to my gsm phone.

Is this related to the codec in some way? Is there some way to switch which codec is used? I have plenty of bandwidth to spare, so that shouldn't be an issue.

It could be an issue related to your wlan. Vote for this bug

Have switched wi-fi power saving to 'off'? Worth a shot, helps with streaming etc.