It is BT that won't allow calls from elsewhere, this is a condition for BT Basic, otherwise people would be applying for cheap line rental and then placing calls elsewhere like with Primus or Call 18185 or even Virgin.
http://www.btplc.com/inclusion/phone...ic/faq.htm#get
....
Even if you're getting one of these three benefits there are certain exclusions that apply,
such as a phone service provided by another company, if you have more than one phone line or have a business, temporary or payphone line.