This wasn't by any chance a migration to an SMPF service was it?
I had something similar recently where I migrated one of my Plusnet broadband connections from Plusnet to an SMPF service and also got an email about ceasing my voice line.
I've been told that this is a limitation of the Plusnet automated systems in that they assume that the line has been taken over completely.
It took many calls and arguments with support to convince them that my voice line was still with Plusnet.
I even got into an "Oh yes it is, oh no it isn't" situation with a member of the options team who was adamant that it was impossible for the voice line to remain with them with a TalkTalk marker on the broadband.
My post on the community site has remained unanswered. From the latest tickets I'm assuming all will be OK now