To make long story short - if you are Voipcheap.com customer (it might apply to other Betamax companies such as Voipbuster, etc) then you can make international call via your VoIP account from your mobile.
It works quite simply - dial 01617002626 from your mobile phone and then dial number abroad. The system will tell you how much you'll be charged for the call.
To make such calls, you need to add your mobile phone to your account. Remember that Voipcheap is pre-pay service rather than post-pay.