This has come up before. Your best option is to leave the BB router as close to the incomming phone line as possible. Putting the (analog) signal through multiple connectors/splitters/low quality cables will degrade it. If wireless doesn't work, use an Ethernet (RJ45) port on the router, and bring that upstaris using CAT-5 cabling. This is far more robust and less error prone than bringing the phone line iteslf up. In terms of the amount of work involved, it's similar to running a regular phone line.
In decending order of choice, I'd say:
- regular wireless BB router and wireless PC (but you've already tried that)
- boosted wireless or improved antennae, as someone else suggested: this can be something of a black art, and you could find yourself investing in a whole series of options, with little improvement. On the other hand, just using better antennae might do the trick, and you're left with the benefit of wireless
- as I suggested, terminate the BB at the point where the phone line enters the house, and run an Ethernet connection using CAT-5 cabling. This is guarenteed to work, and will give the fewest errors and hence highest throuput
- last on the list is to run the phone line itelf, probably through one or more splitters and/or junctions. This might work just fine, but equally if you're already on the outer limits of acceptable perormance due to distance from the exchange, it might just push it over the edge into not working at all, or with deverly degraded performance. On the other hand it's the simplest and cheapest thing to do.