Just because Margaret just sent us the very useful newsletter, with tonnes of stuff about Prague, I decided it'd be a good time to check out Mobile providers in Prague. There are 3 main providers - Eurotel (the Czech incumbent, affiliated with O2), Oskar (owned by Vodafone), and T-Mobile.
My instinct was right, in that these are networks that don't really "play well together" - so if you are making calls between different providers, you get gouged by higher rates. This doesn't happen with the US, but is certainly a concern in say the UK.
So my proposal is that people (if they're smart) will stick to one carrier, rather than going "off-piste". What I've found so far suggests that Oskar is the way to go (and I've always had a good experience with Vodafone in the UK).
Just a direct comparison of tariffs (all pay as you go, of course, by getting a sim card) - Oskar and Eurotel, the pricing seems that much better, and much less complicated, for Oskar - though you would have to take into account the free weekend calls on Eurotel (which I'm always skeptical about - read the fine print below).
Caveats I would bring up would include the following: I have very little idea of the extent to which US phones are locked in to their provider. You may very well find that your phone is unable to use a provider other than the one you're using now. But if your phone is "unlocked", it's just a matter of switching sim cards (please tell me this is an elementary skill, like walking upright and saying fuck).
SMS is also going to be very useful as a communication tool, though calling rates are actually surprising good. What also convinces me about Oskar is that their online SMS sending service has a nicer interface, compared to Eurotel's.
Honestly, I haven't tried the hardest with T-Mobile, partly because they don't have an online SMS system, and because their website is woefully underdeveloped. So much so I can't even find a tariff page.
So yes, I can do a cut down version of this for margaret, for the program, or I could wait for more consultation, or I could set up a tinyurl for this page, we'll see.
Conclusion is very simple. Stick to Oskar/Vodafone. Even in the UK they had over-taken the state-run incumbent.
Edit: to put things in perspective, 1 CZK is about 4 cents (and a bit) in the US, so it'll be about 8c for a text message, 12 cents per minute for a call within the network. Just imagine, that calling outside the network will be about 24 cents per minute and you'll realise why we'll all want to "keep it in the family".