In addition to what has already been said, I would add a warning based on recent experience of just such a transfer.
VM, for me , happily gave an install date sufficiently far in the future to allow for notice to my existing ISP.
Shortly before the date (about a week) they told me they wouldn't be able to meet the date, and set a new date about a month later (due to permission being needed from the local authority to do road works).
At the original date they took over my phone line and broadband, and then rescinded the takeover for the phone line (not the broadband). A few days later my phone ceased to work, and so did my broadband. My old ISP said they couldn't reinstate my service because VM still had a lock on it, and then had to cancel everything because they had now way to get things back.
VM persistently asserted that there was nothing they could do, even tho' they had caused the mess, and my old ISP explained what had happened, and how things worked in the transfer process, so they couldn't get me live again
I never managed to get my old phone number on the VM phone service.