This may seem a funny question, as, how can something that is free be good value?
The issue is that whilst the sim card may be free, the calls are not, and, it is often the case that the call charges are higher when using a free sim.
All the free sim cards tend to be given on pay as you go contracts, and pay as you go are notoriously more expensive on a per call basis. Sure, if you do not use your phone, then it will not cost you anything as there is no line rental to pay, but, if you use your phone extensively, it does not take long for your expenditure to exceed what it would cost you on a monthly contract.
Conversely, on a monthly contract, you are likely to be given a free handset, clearly, if you take a free sim card this offer will not be extended to you, as the network has not idea, let alone guarantee that you will use it to make calls. Therefore, the network has no idea if it will make a profit from you.
In short, free sim cards are a great idea if you do not plan to make many calls with them. They are good for a spare phone. However, if you plan to use your phone in this way, check whether the credit expires on your card. For example, some networks attach a 3 month life to their credit, so, if you do not use your phone for three months, or do not use all of the credit within this period you will lose it!
