while spending a lot of time with my n900, i have found it not very useful as a sip phone because the lack of dtmf tones for a sip account. on my n810 i could call into conferences just fine, but the n900 will not allow me to enter the conference number.
i did some googling and found:
looks like i need to update my os to make this happen.
*blogged from my n900 and fennec*