Here's a hint, if you happen to have access to an ISDN line with a nice phone switch or similar attached to it.

Dial an 0870 number, and watch for the ISDN called line identification to be provided. This is a standard feature of ISDN lines.

Depending on the service provider for the NTS number, you may, or may not, get the actual geographic number returned to you as soon as the ringing signal happens.

Again, depending on the service provider for the NTS number, they might be routed to a number that isn't routed outside their network, so may not be directly diallable as a standard Geographical number.

Hope this helps to find some more of these.