Only any good if you make a habit of calling whilst on the move, or your mobile is your only phone.
Dial thru via a fixed BT line using an 0844 could work out cheaper.

To be aware of - CONSTANT TOP UP
http://www.returncall.co.uk/rates.php
*Once you run out of returncall credit, you will automatically receive a 5 pound recharge. To unsubscribe from this service please text RETURN OFF to short number 81223)

FAQs need to be read
Can I use my Topped Up credit on a different phone than the one I used to Top Up?
Unfortunately not, you can only use your credit on the phone line you have used to Top Up

This service can only be used from mobiles, according to one line. However this line appears also in FAQ.

I am preselected at a different phone company than BT (like Alpha Telecom). They prevent me from calling your access numbers. What should I do?
Dial “1280” prior to our access numbers to bypass the network of your current telecom provider.