Sims Card to buy

First, it's SIM, for "subscriber identity module;" SIMs is plural.

In London you can go to a Carphone Warehouse, where they sell SIM cards for multiple carriers, and explain your needs; they can then talk you through your options. Since all of your countries are in the EU, the roaming costs are capped. However, it can be difficult to top an a card outside the country where you bought it. My friend has a T-Mobile UK SIM and he can top it up online using his American Express, but another poster here said that his US-issued Visa and Mastercard would not work for this. So if you really want to use the same SIM for the whole trip, put on a lot of value while you're still in London. Or, you can buy a SIM in London, using up its value, then just buying a new SIM when it runs out (in whatever country you're in at the time).

If you want to do more research in advance, use PrePaid GSM. Here's their page of carriers in the UK: http://www.prepaidgsm.net/en/uk.php and here's their UK and Ireland forum: http://www.prepaidgsm.net/forum/forumdisplay.php?f=38. Be aware that discussions in that forum can get very technical, so don't be afraid to ask for clarification.

Do you want data as well as calls?
Do you want to be able to call back to US from all 3 other than an emergency call?
Are he time splits about equal?

We've made a couple of trips to England over the past 2 years. Three has had the best deals on prepaid phone sims which include data--$25 for plenty of minutes and data. Carphone Warehouse is a good place however I don't know if it carries Three. The store near Harrods didn't last year. I recall the Three call roaming rates outside the UK were very reasonable but data roaming was expensive. If you don't need data I expect CW will have what you need at a reasonable cost.

Oops! Thanks Harold for the correction.

sally,

You could also consider buying a SIM card from one of the travel phone firms, such as Roam Simple, Cellular Abroad, Mobal, Telestial, EuroBuzz, Lebara, One SIM, Max Roam or others. One advantage of that approach is that you'd have a working phone as soon as you step off the plane, and since you already have phones that would be an inexpensive option. One firm is advertising Europe SIM's for as low as $5 at the moment. Check their respective website for prices and the "fine print".

I'm assuming that your phones are unlocked quad-band GSM models? Are you using Smartphones such as iPhone or Android? If so you'll have to deal with the aspect of data charges.

Many of the travel phones use U.K.-based numbers, but have fairly consistent rates across Europe. The rate structure is arranged in "zones" and the costs may be slightly higher in eastern Europe and Switzerland. Many of these use post-paid billing (calls charged to a credit card) rather than pre-paid, so no concerns about topping-up. As Harold mentioned, that could be an issue outside the country where the SIM was purchased.

While you could also buy a SIM card in each country, that's a darn nuisance especially as the phone number changes each time you change SIM's, and this probably won't be the cheapest option.

Thanks Harold, Ken, Lee and Richard for your feedbacks. I have an Iphone 4( will update this July) under Verizon. I am planning to get 100 MB of data for $ 25 from Verizon before I leave . Then get the SIM card with more minutes that will hopefully last till Venice and Prague. I was told that there is an App I can download call Viber to call home for free as long as there is free wifi.

Note that a Verizon iPhone 4 will only get WiFi in Europe. It does not have a SIM, and you cannot get voice, text, or data roaming in Europe. However, a Verizon iPhone 4S will get all of these in Europe, as will the later models (5, 5C, 5S). If you are getting a new phone and it's not an iPhone, check to be sure it will work in Europe, and that the SIM is unlocked.

sally,

Hold the phone!!! As Harold mentioned, your Verizon iPhone 4 will likely NOT work in Europe. You'll have to "go back to the drawing board" on that point. Under the circumstances, the easiest option would be to pack along the iPhone 4 to use in Wi-Fi areas, and buy an inexpensive basic travel phone from one of the travel phone firms I mentioned above ($29 plus $5 for the SIM, along with usage charges).

Regarding Viber, I believe that anyone that you wish to communicate with must also be using either the phone or desktop version of Viber. If this is used outside of Wi-Fi areas, you'd be using 3G data which would likely be costly when travelling in Europe.