Questions about the new MP58 phone

[jaseinatl]

I am assuming that MP stands for Microsoft Phone, but I am unclear whether this is a requirement or just a predisposition. In other words, can the MP58 be used for SIP?

Also, We have been reluctant to leave our existing Mitel 5630 phones because of one idiosyncratic feature that we haven't been able to find anywhere. A CORDLESS Handset (not headset, not DECT)--no screen or fancy features. Just like a regular, desk phone handset without the cord. Once you get used to it, it is nearly impossible to go back and not be disgruntled.

My question is if I can't get the MP58 with the cordless handset (new) to run with my VOIP provider NEXTIVA or is there any way to purchase the cordless handset with a dongle or just use it with Bluetooth on the T58A?