Jacktho:
One problem is that sales reps in the US (for all the companies, not just T-Mobile) are very often not knowledgeable about how their phones and plans work with international travel.
I have spoken with T-Mobile salespeople who have actually traveled to other countries and used their phones; what they say matches what T. says above. If you have one of their Simple Choice plans, you do not need to spend more to get the good international rates when you travel.
The $10 additional is to get good calling rates FROM the United States TO foreign countries (it's $15 to get good rates to more countries and to cell phones). It does not apply if you are using your phone in Europe.
When you take your T-Mobile phone to France (as long as you have a Simple Choice plan), you get unlimited 2G (slow) data, unlimited texts, and calls at $0.20 per minute (whether to the US or within France). However you retain your US number.
The good news of this is that people in the US call you as normal, and it doesn't cost them any extra. It costs you only $0.20 per minute to receive their calls.
The bad news is that for someone in France, calling you means calling the US. They would dial +1 then the area code then the number, and would be charged for a call to the US (depending on their plan, this may or may not be affordable for them).
The other bad news is that to call a French number you must do it as an international call (still only $0.20 per minute). You dial +33, then the last 9 digits of the French number (omitting the initial 0, but including the rest of the number).
By the way, you can program your numbers in this way, and they will go through regardless of where you are. In other words, if you program in Rick's office as +1-425-771-8303 (without the dashes), you just need to press Send on your phone, and it will dial correctly wherever you are.
If you want to have a "local" French phone, you would buy a French SIM card and put it in your phone (if it is unlocked) or buy a French phone at a local store (if your phone is not unlocked). T-Mobile is good about unlocking phones once you have met their terms - call customer service to ask how to do this for your phone.
If you have a French phone, everything I said above is reversed. Calls within France are local; calls to the US are international (but some SIM cards have low rates for these calls). A French person calling your phone makes a local call (cheap and easy), but an American calling your phone makes an international call (potentially very expensive, if they don't have some kind of plan - such as the $10 per month T-Mobile plan!) However, receiving all calls is free on a French SIM.
I know - clear as mud. Ask if you need further clarification. But for your needs, I think using T-Mobile is best, unless you have a lot of people in France who need to call you. And if you do, they can text you (international text is cheap), and you can call them back at $0.20 per minute.
