1.) Full migration has not completed: Although your phone line is supposedly activated it can take until midnight to fully go through so this could be one cause.
2.) You have a phone line fault No dial tone is a type of line fault in this case an engineer would have to be sent out to correct the problem.
The only really way to check what is causing this is to contact plusnet which you can do at: www.plus.net/contact alternatively there are plusnet staff on the forum who could review this for however they only work office hours so I can't see them picking this up for you today.