Each cellular phone would have to have a different number, but one of those could be your current landline number ported over. Prepaid service might be an end-run to keep such a system affordable though.

What I do not understand is why, if your landline service truly stinks, you don't utilize an inexpensive system like Ooma, which is basically an IP phone service that runs over your home internet connection. [Just one example- there are plenty of other decent IP phone services.] This way you would keep one 'home' phone and one cellular. The one time up front costs for the hardware can be <$79-125, and monthly service charge as low as $6. Much cheaper than your (mostly taxed) landline and also very reliable/stable. Also, for an additional charge you could even port over your old (familiar) landline number to Ooma. See here:

http://ooma.com/products/basic

http://www.frys.com/product/6976017