You need to make a distinction between SMPF (shared) and FMPF(full)
Talk Talk is rare in using fully unbundled and the way to moving to another provider involves getting your phone line back to a BT line or equivalent and as such the procedure does not need a MAC.
For shared services e.g. Be, UK Online, Sky then a MAC is available and should be provided.
Another note the line can be taken back to WLR providers such as The Post Office or Madasafish who place new line orders.
Process for leaving full unbundled e.g. Talk Talk is
Simultaneous Provide
The process has existed for some time, but not all providers have adopted it yet. It involves combining an order for a telephone service and broadband service with potentially two different providers and then marrying the two orders together using a LOR number (LORN) so that they get connected up at the same time.
Generally people start the process by contacting their chosen broadband provider and ask to do a simultaneous provider order, the LORN is supplied which can then be taken to the voice service provider and used to order the telephone line. It should be possible to order the telephone service from any provider who uses a Wholesale Line Rental product, in addition to the obvious BT Retail products.
Some providers who have not adopted the latest ordering systems may an older system which is being phased out which involves ordering a PSTN migration and once this is working order broadband again from new.