You have to know which network the other person's phone is on, too. I got caught out that way once, knew the number but not the network. However I managed to get there by trial and error: at least the machine told me that I'd picked the wrong network, it didn't just 'take the money and run'...
I believe it's quite straightforward to top up from credit/debit card when abroad, if you know the right service number. But I usually set out with plenty of credit on the phone, so the need doesn't arise.