You will need an Outbound Route that sends those forwarded calls to the analogue trunk (the "POTS line" as you call it). Can all calls to cell phones use the POTS line or do forwarded use one trunk but direct calls use a different trunk?
Once the outbound route is configured it should simply be a matter of dialling *72 followed by the cellphone number you are forwarding to (dial it on the phone you want to forward). To cancel call forwarding, dial *73 on the same phone.
By the way, I agree that the question is not very clear - I am guessing that I've understood correctly what it is you're trying to do.
