Voipfone is another supplier. I have used them for the past 8 years.
There is also the issue of hardware. For Dect phones, the Gigaset N300 base station works well with Gigaset phones. For wired phones you need an ATA such as the Grandstream HT801 or the Cisco ATA 191.
The suppliers mentioned should be able to help you configure these devices
Your ISP of choice will supply a new router with ports to plug in an analogue telephone phone which could easily be your existing hardware.
No, *some* ISPs do this. This includes BT, Sky and Vodafone. I presume EE too, but definitely not Plusnet. Talktalk only do it for FTTP customers AFAIK. Zen do it as an add-on service.
If you go with a completely separate VOIP provider, this has a bunch of benefits:
* You can change broadband provider as often as you like, without risking losing your number
* You have a wider choice of ISPs available, including those which don't provide any voice service
* You are not locked into using the ISP supplied router
* You can make and receive "landline" calls when away from home
* You can move to a different area of the country (or even a diferent country) and keep your number