SIM for VOIP

Backstory: I am about to lose my ADSL Broadband.  It's clear that, sooner rather than later, Openreach will 'convert' everyone in the area to fibre.  For several reasons it makes more sense for me to opt for mobile (4G) broadband now; there's a good signal at the top of the house even though there's a poor or no mobile signal over much of the rest of the house even though my 3-handset cordless phone system works perfectly throughout the house and garden. I need to keep the cordless system for personal safety (75 and living alone) and I've found a 4G router with an analogue phone port.

Does Smarty support VOIP?  If so do I need a Voice SIM or just a data SIM?