My calls are forwarded to my cell. After I receive a call on my cell, how do I forward that call to the phone at the office?

I typically advise users to use the RC Phone app on their cell instead of forwarding. There are multiple benefits to this, including better identification of work calls, the ability to send and receive work texts on cell, and the ability to disconnect from work by logging out of the app.  In addition to these benefits, you also get the ability to use Call Flip to easily switch the call to your desk phone or desktop app.

Beyond that, if the call is 'live' on your cell, then your mobile provider should have instructions on how to forward calls.

You won't be able to just forward the ongoing call from your cell phone to your desk phone or desktop app. If you were using the RingCentral mobile app you would be able to FLIP the call to either of those locations.

If the call is on your cell phone via your cell phone provider, the call can only be forwarded before the call comes in. In other words, you would forward your cell phone calls to another number. You can't forward a live call on any provider that I am aware of.

Bob is correct....try using the RingCentral Mobile app and avoid that issue altogether.