Taking Cloud Voice to a new ISP?

I am helping a village shop, who have foolishly signed up to what seems to be a three-year contract for Cloud Voice. £30.73+VAT per month for two handsets and UK calls seems very expensive for what they need.

That is bad enough, but the prices that BT are quoting for SOGEA are pretty horrible too - especially if I only want a 3 year deal so that both contracts end around the same time.

If we are stuck with the Cloud Voice contract, are we automatically stuck with BT internet too, or is there a way that a technically competent person and perhaps some extra kit can allow us to use the Cloud Voice service for the remainder of the contract, over another ISP?

The BT Cloud Voice FAQs state

Do I need BT Business Broadband to use BT Cloud Voice?

No, as long as you have sufficient bandwidth you can use BT Cloud Voice. We can discuss your current set-up when you talk to us.

Got that from here.

Seems you can use any capable broadband solution for it (the risk is that if you have any issues then BT and the ISP might end up playing ping-pong and just saying the issue is with the other party).

When they renew I recommend Vonage.
I don't use the business plan but the home service is very cheap.