An answered call? As in you picked up the handset, said hello, and need to transfer it to me on my cell phone?
Press Transfer, dial 9, dial the cell phone number, press # to speed up dialing or wait for the call to connect, and then press Transfer again

If it is someone that you transfer to often, create a phantom extension number that is permanently forwarded to the cell phone, then you can just Transfer, extension number, Transfer.

In all cases, the lines have to be T1/PRI or Loop Start with Reliable Disconnect, and the extension making the transfer must be enabled for Trunk to Trunk transfer