Is there a service that can help me ?

Well I am very new to VoIP and have very little idea about it.

I wish to know that if there is a service that can provide me with a USA telephone number, and then forward the calls to my o2 mobile in UK. This way my family and friends in USA and India(using magic jack), can call me for free.

Well if there is any such service, then please let me know about it.

Also I would like to know if such service would be free or I have to pay a fixed amount monthly or I would have to pay based on the number of minutes I speak on phone.

Thanks.

Calling UK mobile from abroad (including the US and Canada) is very expensive. It is relatively inexpensive to call the USA via landline and it can be free it you use sip to make the calls. You could organize it so that yalls to the US number would be then sent on via sip to the UK for a small cost. I expect you cell provider gives you some (you paid for them)free calls so it is probably better for you to call them and if they can accept the calls using sip (eg. perhaps via Voipuser) at a low cost. Can your friends receive sip calls?