Simple VOIP solution to replace landline

My sister-in-law lives way out in the countryside in Wales and ... can now signup to fibre broadband. But she doesn't want to be without a landline. What would be the simplest solution to this? A VOIP phone where she has one deal with the broadband provider that covers both? Or is it best to get the broadband and VOIP separately? She needs something that's straightforward as neither she nor her husband are techies. Some examples would be helpful. Thanks!

Does she actually want to retain the physical landline, or does she want a "PSTN" number and handset rather than a moblie service?

Requirements for it to be straightforward to me says get the phone and broadband from the same company, where the phone line plugs into the router. Coming up to Black Friday as well so it's a good time to be shopping.

Requirements for it to be straightforward to me says get the phone and broadband from different companies, where the phone line plugs into an ATA rather than the router. Get the phone service from a VOIP only provider.

Doing it this way, you can change phone provider easily and also you can change ISP without worrying about losing your landline.

Ok, what do you do if you have problems and your VoIP provider points to your ISP because the router took a firmware update and changed how it processes SIP packets.

The latter - just a handset. It doesn't make much sense to me - she has a dumb mobile phone but wants the comfort of a landline.

Do you have any examples of where broadband and a VOIP phone are offered by the same company?

The two simplest routes are

sip ata, where you can use your existing handsets and just configure or get a preconfigured ata, so plug in and go.

or

something like an isp based service built into the router so its just a case of new handsets.

Some years ago I wrote up how I replaced all the VOIP phones in my office. How I chose handsets etc. All the basics are still the same. I am even paying the same monthly price! Hope this helps:
https://www.sheffnet.co.uk/2017/05/29/voip-phone-sys...

1. Get fibre Broadband installed and working.

2. Buy a Gagaset N300A-IP with handset

3. Migrate the old landline number to a VoIP provider - I use https://www.voipify.net/ who I would recommend and then cancel the existing broadband. Migrating the number may trigger a cacncellation - but just make sure.