lol, all I had to do on the N810 was GV + Gizmo5 (which GV natively supports) + Native SIP client. I recommend looking up the old guides, it might work for N900 also.. However if you want DTMF tones the N810 SIP client could not do them very easily (but could) and the superior FRING app could not do them at all, making it impossible to receive incoming calls!