This seems to be a rather odd area. I have a friend that has a spotty system, where once in awhile his DSL goes down and he needs a dialup line to kick in when this happens. I've spent some time looking for devices online and there doesn't seem to be a whole lot in terms of routers with RS-232 or dial-up failover. Most of the new stuff all has 3G/4G failover. But the big carriers that are supported all try to get you in on their monthly fees. The DSL account for the business he's working for apparently has a free dial-up account if their DSL goes down, so he'd like to utilize that if all possible.
It seems as though routers with RS-232 ports have all but disappeared, same with dial-up.http://www.draytek.co.uk/products/comparison.html
I found those, but they apparently don't work in the US as US dialup is different from UK or something like that.http://www.amazon.com/Cisco-CISCO1811-K ... B0009VPWZS
That is also something I found, but it seems to definitely be niche and I'm not sure it's designed exactly what he's looking for.
I know there are also failover routers, that switch to a different ethernet port once the main one goes down, but you can't find dial-up modems with ethernet ports on them or a networked rs-232 device. They do make USB modems, but simply plugging one into a router with a USB 3G failover probably wouldn't work (I'm going to say most definitely). There seems to be no in between here.