Over the years we have changed ISP's a few times and I realised that porting a phone number each time was going to have problems at some point.
Instead of retaining our long-running number, which only seems to get spoof calls anyway, I set up a third party VOIP number and bought the ATA to connect it to my home broadband. I've actually unplugged the 'real' telephone as a 'new' number provided on the last ISP change was getting junk calls I didn't want. The VOIP line to date has not had one spoof call.
Additionally, I've an app on my mobile that will let me answer and make calls on the VOIP line. My family tend to use, and call, mobiles rather than the landline. Handy if I am out as I can still take the occasional call from my doctor or whoever.
Having said all that, it should be possible to port an existing number to a VOIP provider (fixed or mobile) and do something similar. The advantage being that if you do change ISP again you don't have to change phone number any more. Same as not using an ISP email address and either getting your own domain name or using a free online email service.
Sticking to one ISP these days isn't a matter of loyalty, after a 'new customer' offer expires you often get the price bumped up and little incentive to stay. Moving your number to a mobile broadband provider serves to tie you in to their service for the future.
There are some free VOIP services out there but I pay my provider a nominal £1.20 a month.
EE Fibre Plus