If I'm standing in front of a parking sign that says I can pay by mobile, is that still true if my phone doesn't have service from any local provider? Or is it still true if my phone has prepaid service from a local provider? How do I know? Who do I ask?
That last question is the hardest part, because the customer service lines provided generally don't know what to do with foreign phones. Often they tell you to go ask someone else, like your phone provider, who is likely to know even less about some random local service in a country they don't even operate in.
None of this is what you want to deal with when all you want to do is board a bus, make sure you don't get a parking ticket, or even just get a can of drink from a vending machine. So far I've just given up and used cash or some other form of payment, but this may not work forever as many of these services find payment by mobile phone a cheaper alternative to other payment methods.
I wait for this to become seamless, but I expect I'll have a long wait.