4. Can I keep my existing phone line for voice calls?
That depends on what product you're installing and from which Communication Provider (CP).
If you take a fresh FTTP service, then it's completely separate to your existing phone line and/or FTTC/ADSL broadband. In fact, some providers will only supply a data FTTP service without voice (e.g. TalkTalk)
If you take FTTP from BT and buy it as an upgrade from an existing BT FTTC+voice service, then they *could* keep the copper line for voice, but these days more commonly they will migrate the voice service to an analogue telephony adapter port on the SmartHub that they supply. Some other providers (e.g. Sky) do this too. This forces you to use the CP-provided router instead of your own.
You should be aware that the analogue telephony network is now in the process of being shut down, and it will be completely gone by end of 2025. So you're going to have to bite this bullet sooner or later anyway. Even users who still have copper lines in 2025 will have their voice service migrated to VOIP over FTTC or ADSL.
An approach you should think about is as follows:
1. take a separate VOIP service from a provider like Sipgate or AAISP. Initially you will get a new phone number. Buy your own ATA or DECT hub and test it all works.
2. install your new FTTP as a separate service
3. when you're happy that the VOIP is working over FTTP, request your VOIP provider to port your phone number
4. when the phone number port completes, this will automatically cease your copper phone service, including any attached FTTC or ADSL service that runs on the same line.
It's a bit more expensive as you have to buy your own VOIP device, but it completely decouples your voice service from your broadband. You can change broadband providers whenever you like without affecting your voice; you can accept calls on your landline number via your laptop or mobile; and you can keep your phone number if you move house to a different part of the country (or even a different country entirely!)
Do bear in mind that your broadband (and hence voice) will fail if your building loses power, unless you also have UPS backup power. If you have a mobile and good indoor coverage, that's probably all you need for 999 calls. If not, then consider buying a UPS.
As I say, everyone will be in this boat by the end of 2025 anyway. However if you're forced over to VOIP by your CP, and are also classed as a "vulnerable" customer, then your CP will be required to provide you with a UPS which can provide at least 1 hour backup power. Some CPs (Sky at least) are currently refusing to migrate people over to FTTP in this situation.