I've not seen a smartphone app specifically for that. Of course, many soft phones have some kind of call forwarding.
OTOH, I've used a GSM gateway device to make a T-Mobile account available to my IP phones as a trunk.
That was fun. It would be even better now if such gateways supported transcoding AMR-WB to G.722. Perhaps a GSM module on a TDM card in an Asterisk server could do this?--