All you need to do is set up call forwarding for busy and no answer. The common cell companies have easy ways to accomplish this, by simply dialing special code with our number as part of the code. Additionally, some Android phones will let you do this right from the phone!
T-Mobile and ATT: Dial *004*14806464562#
Verizon: Dial *7114806464562
Sprint: Dial *2814806464562
Right from your Android phone!