Do you have anymore information you can provide Gimitel? I have the bria sip softphone on my iPhone as well and tested a call like this:
ss4025: Dialed the softphone
Bria: Answered, put on hold, txrd to 5360ip
Success...I let it ring 3 times
One thing to note here is that all three of the above extensions live in the same Mitel 3300.
The transfer should be working though, check your Class of Service options to see if there may be some timers that need adjusting?