I have a BT Line and one with NTL (not showing off, the latter came free with cable installation
). On the BT Line, I arranged for a so called "carrier override" with One.Tel, it means that all calls automatically go through One.Tel saving me the dialling of the 4-digit access code. On the NTL line, I connected a small gadget from Superline, called an "auto-dialler". It basically does the same thing, except you simply unplug it if you want to make calls directly thru NTL.
Calls to the US (home and cell phones) with
Superline are 2.5p (incl. VAT) per minute, no monthly connection charge. They also offer a monthly plan called "Supercall" for £12.99 that includes 1200 minutes for national AND international calls which equals to a call rate of 1.8p per minute. The plan is currently only available for BT lines.