You may find that you don't have enough telephone points in your home - sure, you can use cordless phones and wi-fi, but what if you need an extra socket, perhaps for your Internet, a fax machine, or maybe for your Sky set-top box?
If you want to get BT to do the work for you, you can call BT and ask them to come and fit a telephone extension socket for you. Their website doesn't actually specify a price, but asks you to call to discuss your requirements. Be aware that unless BT is doing any other work at your property at the same time, their standard callout fee will apply. Last time we checked, BT will charge £116 for a site visit.
A typlcal extension for us to fit usually falls within the 2 hour price rate including the parts.