|A variation on what you're asking is ISDN and was widely used in 90's. In fact, until about 3ys ago, our company used it as our backup connection to our stores if/when the T1 connection dropped.|
You can still find ISDN ISP providers, but not ones that support modem teeming. Keep in mind that ISDN is going to be much more expensive and a lot slower than current broadband (DSL, cable, or satellite) providers.
ISDN ISP Providers