@ilf

1 - check your wifi settings, maybe disabling powersave could help (on both android and N900)
2 - install a reliable sip client into android (Csipsimple works great)
3 - if you have a low bandwidth or high latency connection use gsm(8k) / g729 _NOT_ g711a/b
4 - if you know what you are doing, check your sip server nat and sip connection tracking (and/or stun server setting)
5 - check also your provider about complaints of someone else about QOS on sip calls (today it's a very common behavior to limit voip).

6 ..good luck!