If I forward calls from a desk phone to a cell phone, what CID numbers will be displayed on the cell phone when it receives calls forwarded from the desk?

That depends on what system you have and what kind of trunks. Most likely it would show the trunk number of the outgoing line.

I would think it would show the Main Number of the site

It's a Mitel 5000HX.

Would it work during Dynamic Extension Express or on a PRI?

I just ran into this on a comcast pri.We could outpulse the main number but not the number that called in.The numbers that came in are not part of the range of did's so it would block the call from going out from comcast.