Yes, the message varies. I've just had one saying unable to verify at the moment, try again tomorrow.
For this one, it was a retun to Talk Talk and they sent me a pre paid sticker , and a link to book a collection. When I tried to enter the number I got a similar error message, so on Wednesday I went to the PO and they beeped it fine.
I asked them if number was okay, mentioning problems with booking it online, and they sait the munber is fine, even comfirmed the open "OT" rather than "0T" (It has to be two letters, then nine numbers then two more fimal letters.
Thanks for your help