This article explains how to find the phone line number associated with your broadband connection. If you have a broadband-only service, you may still have an active phone line number assigned to your line.
Find your phone line number
Connect a phone handset to the same socket your broadband router is connected to. If you are using an ADSL microfilter, connect the phone to the microfilter's phone port.
Dial 17070. The automated service will read out the phone number your line is activated on.
Please note: 17070 is a BT line test facility. It is free to call and works on most UK landlines. You will need a phone handset that can connect to your wall socket to use this service.