I had terrible trouble in the Western Isles with EE, who are the only provider in some areas. None of us were fully familiar with PAYG but I had some experience.
My friends wife did the shop run and bought a SIM, shop didn't mention that a top up voucher would be good too.
Turns out their PAYG top up number was not working on the local tower. Worked fine on a different tower later.
Internet worked at least to their website but instructions provided didn't work.
Eventually we "wasted" a day to go back to the shop (it was the only one around) and get a top up voucher. Top up via TXT worked seamlessly. Hardly wasted since it is a beautiful part of the country but it was not on the agenda.
I couldn't believe how hard it was to give them some money.