I am absolutely certain you can port your number in the same way you can with a mobile. Obviously, you can't port it to a copper lined analogue supplier who uses the Openreach Network due to a stop sell of all all-copper products from Openreach.
At the moment it's all on copper - which is what I mean by analogue. I'm being told that some time in the undefined future (but before 2027) it'll go digital whether I like it or not. BT will convert my current analogue phone number to digital but when that happens I'll lose my plusnet interweb access.
Now of all things, the phone number is most important to me. Had it for 50 odd years and don't want it changed. Can't move it to anyone other than BT without moving the interweb at the same time - because I would have to move them to an all digital supplier. I'm strongly of the oppinion to only change one thing at a time, because when (not if) it goes wrong you know who to shout at. So I don't want to risk changing phone line and internet suppliers at the same time. I don't use any plusnet services - email, web hosting etc so losing them isn't a problem. Therefore AFAICS the safest sequence is...
1) Move the interweb to BT, leaving the analogue/copper phone line alone for now.
2) Wait for BT to downgrade everything to fibre/digital
3) Once safely downgraded, shop around for the best deal.
You'll be hard pressed to get a BT anything for the next few weeks. EE is (currently) BT Group's consumer brand, along with Plusnet. BT is currently the brand exclusively for Business customers. Except those that are on legacy BT products. A mess? Yup, a proper foster cluck nobody understands.
However, you can take DV (the generic name for phones provided over the internet, as opposed to POTS, the name given to old fashioned analogue phones) from many suppliers independent of the Internet connection. Much like taking separate electricity and gas suppliers. My DV is not supplied by either of my 2 internet providers.
As for the demise of analogue telephony in the UK, it's inevitable. GPT's System X is a product of the 70s, and mostly deployed in the 80s and early 90s. Ericcsion's System Y is marginally newer and used in the UK after GEC and Plessey (2 of the 3 System X suppliers) merged, and British Telecom didn't want a single supplier. As it happens, GEC ended up being sold to Ericsson anyway, so still a single supplier 
.
This stuff is old, originally designed for a 20yr life, and most is 35-40yrs old. Parts at a component level are getting scarce and expensive, they aren't the most energy efficient, and need updating. Trouble is, there aren't a lot of options, and the move to IP is a viable option, with the benefit of not needing a telephone exchange every 5 miles.
There is a short term transition system being fitted in exchanges that still uses the copper lines to the customer, and it does the POTS to IP at the exchange itself. The disadvantages of this are that the Nokia equipment being deployed goes out of support in about 5yrs, still requires an exchange every 5 miles, complete with the building, batteries, generators and so on. Hence it really is a transition solution.
DV tends to offer advantages in voice clarity due to better bandwidth, modern codecs and so on. Its disadvanatages are the customer has to provide the power (no 50v from the exchange), and most people's phones will need an adapter as they are not DV native. They also won't work in a power cut unless the customer has some kind of battery backup for the adapter and the router and the ONT (if separate items).
Hope that clarifies 
Yeabut, AIUI if I attempt to sign up for 3rd party DV (or SIP or External adapter), it's highly unlikely I'll be able to keep the same phone number, because at the moment my (plusnet) internet relies on the copper BT phone line.
With the existing copper setup you need a working phone line to use the internet, whereas it's the other way round with DV - you need a working internet connection to use any form of DV. So to sign up with a 3rd party DV I need digital internet first. Plusnet don't offer DV at all, so the first thing that needs to happen is I move my ISP to one that does support DV.
Plusnet (a division of BT) suggest I switch to EE (another division of BT). The letter I have from BT threatening me with the downgrade says I should switch to "Fibre 2" on their BT Customer Exclusive Switch and Save. If I check on the BT website, it only offers me BT Fibre 1 - which is a lower bandwidth connection though bandwidth isn't important to me. I live at #34. If I check #32 they also only get offered Fibre 1. If I check #30 they get offered Fibre 2. Yet we're all off the same telegraph pole - I can see the wires going to each house from the pole opposite from the window - infact I'm closer to the pole than either #30 or #32.
I have zero trust in BT after the experiance changing to DV at work - took 2 weeks to sort out the internet and we had the wrong phone number for nearly a month. I also think there is a noticable 'satelite delay' in the voice comms that wasn't there when we were analogue/copper.
However, on balance and under the change one thing at time so you know who to shout at principle, I still think changing ISP from Plusnet to BT is the way to go.
