It is important to read the terms and conditions that come with each card to understand the fee structure and call charges. You could accordingly calculate the approximate per minute charges for destination numbers you are most likely to call. The charges depend upon where you call, source of call, whom you call and how you call. These factors are personalized so you need to make your own calculations.
1. Where You Call: This is the country or the state to which you are making the call, the destination of the call. The tariff to each different country and each different state within the US are different. If you frequently call UK from USA, it is wise to choose a card that offers lower rates to UK calls.
2. Source of Call: This means the means used to make your call, whether from a cell phone, a regular phone or a pay phone. The source of call also has differential tariff. If you have a regular phone and intend to make calls using it, try and find a card that saves cost by using a regular phone.
3. Whom you call: This specifies the phone you are calling, whether a cell phone, a regular phone or any other kind of phone. The tariff varies widely fro regular and mobile destination phones. Make sure you choose your card wisely depending upon the calls you intend to make.