Many thanks. We've been paying Plusnet about £29 per month including all UK calls for FTTC 40/10. But the contract is ending, and the "best offer" so far is £40/month for the same service. Then there will be yet another RPI+3% or so increase next March.
The problem is that you are paying for UK calls, so they think you want to keep the home phone service.
Plusnet's current broadband offerings are without any voice service at all (SOGEA line, and no digital voice offering). For new customers to that service it's £24.99 currently for 40/10, and they'd easily match that for an existing customer.
Vodafone is about the same price, but includes a digital voice service on the router (although a call package would be extra). Talktalk's offerings include analogue voice.
As you've discovered, you can get unlimited voice calls and texts on a mobile phone for under £5 per month, so if your mobile coverage is OK (or you operator supports wifi calling) then you're better off doing that.
It then remains what do you want to happen to your existing landline phone number for incoming calls. If you don't mind losing this, then obviously there's no problem. If you want to keep it, then either you need an ISP who provides digital voice, or you need to migrate it to a VOIP provider.
We can get a data SIM with enough data for well under half that and then tweak our mobile service to get unlimited calls and abandon the wired line for very little.
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But will the service be acceptable? See original post.
That very much depends on which network you go with and what their coverage is like in your area.
Even if you have very good coverage from a nearby mast, it's not uncommon for mobile operators to switch off a particular mast for weeks at a time for maintenance, at which point you could end up connecting to a very distant mast with very poor performance. Also, shared mobile spectrum is much more likely to become congested at busy times.
So if you don't mind suffering that for a few weeks a year, and saving money is more important than reliability, then it may be OK.
Personally I would take a stable wired connection over a potentially flaky mobile one any time. Having said that, faults in copper-based services are not uncommon either. Gross faults are generally fixable in a short period of time (and most ISPs offer compensation for longer outages), but slow degradation over time also occurs, due to crosstalk and other issues, and you have to live with that.