My understanding is that there are a couple of misleading statements within the TalkTalk help pages, referring to the username and password and giving a number to call as you said. They used to be required, but they are not now, it changed a while ago but the help page has not been updated.
TalkTalk authenticates your connection automatically just using your phone number, according to a recent post on the TalkTalk Community - I don't have a link to hand.
Have you tried leaving the fields blank? If the router needs the fields to be filled, anything at all will do, a random word or number should be fine.