First off, you'd forward to the same digit string you'd use to dial from your phone. I.e. 915556667777
Having gotten that out of the way, we'd need to understand if it's a problem with your cell.
Try forwarding to any other line, perhaps your own number or main number - just not a cell.
I don't expect your cell phone to be a problem but just in case, it's the easiest test.
What I've encountered many times is that it's a COR or COS issue.
In COS be sure public network to public network is enabled.
If it's a COR issue, the call will be using the incomings COR, not the forwarding device.
An easy way to test this is to create a system speed dial number that forwards to the cell. Be sure to tick "override toll security." Then forward to the system speed dial.
Ralph