maemo.org - Talk - SIP forwarding/transferring?

My terminology is probably messed up here, but I've been using OnSIP with my N900 for a couple of weeks now and it generally works pretty well as a barebone client (no 3 way calling or call forwarding from what I can tell).

One nice feature OnSIP has is a Firefox addon that converts phone numbers on webpages to click to call links. When you click on the link your sip phone rings and when you pick up it is supposed to transfer the call to the phone number that you clicked on. Works great using a sip desk phone, but it doesn't work in the N900. The Nokia rings and I can pick up and the call stays active, but I'm never transferred to the number I'm calling. I just get silence.

Here is how OnSIP describes the process.

Quote:

The plugin is built to be completely independent of the phone that you use. The call setup is done by placing a call to your registered phone(s) and then transferring you to the destination number that you originally dialed once you answer.

This lets us integrate our call-setup service with any compliant SIP phone on the market and is in fact a standard way to implement third party call control in SIP.

Any idea why this doesn't seem to be working with the built-in SIP client, and thoughts on if it would be possible to make work?