Bundle Lines for DDR Routing?

We have a remote site connected to our LAN via a microwave connection. We want to use Dial-On-Demand Routing (DDR) to establish a backup LAN connection in the event of the microwave failing. We can get regular POTS phone lines installed at the remote site, but not ISDN or DSL (it’s *too* remote, according to the telco). Is there any way to ‘bundle’ several of the phone lines together to up the bandwidth, instead of just using a single line? How would it work? Any other suggestions?