I have used the Roam Simple World SIM card. I did some research on many World SIM cards and phone services that are prepaid and then compared the rates and service offering to Roam Simple
Basically, you can now get global SIM cards that let you pay for your call
credit in advance – prepaid. Their advertised call rates are also quite low and offer free incoming calls in many countries. However, there are a number of disadvantages of a prepaid service. And global prepaid SIM cards have an extra disadvantage — they don't allow you to direct
dial.
This is because, to be able to give you cheaper call rates in certain
countries, they had to structure the way you dial differently and introduce
an extra step that makes it quite complicated.
Each time you wish to call someone, you have to call a third party number
first, then follow the prompts to dial the actual number you want, and then
hang up! You then wait for a call back from this third party, which connects
you to the number you want. Its called a callback service…its a dirty alternative to international roaming.
Yes, you may get slightly cheaper calls, but it can be at the expense of actually being able to make and receive clear and reliable calls. Which should be the most important function of your service.
On the other hand, the Roam Simple World service is post-paid, and works in the same way as a domestic cell phone service does without the contracts and monthly fees. Roam Simple services bill you only for the calls you make and directly to your credit card. It's simple and convenient.
The advantages of using the Roam Simple World servicein terms of call quality, it's much better than the prepaid style service, as you're dialling direct to the number you want to call, with no third party system in the middle.
If ease of use and call quality are priorities for you, the Roam Simple World service is your best option.