I'm not sure how magic jack would work out at the remote end but i've had issues with dtmf being passed when calling a sine systems remote from a magicjack line.
if you just need to extend an existing line somewhere out to a site with internet access, linksys makes some voip adapters that when used as a pair can be configured to bridge a line over any ip connection complete with dtmf and everything.
What we are going to do here is get a voip service with a hardware adapter and put at the transmitter site, which will connet over the t1 from the studio then to the internet from there, saving a good deal of money over a business landline.