If your phone has a hardware issue causing the WiFi drops, then the cheapest way to make WiFi calls would be to buy the cheapest phone possible to support that need. Virtually any smart phone on the market from the past few years can meet that requirement.
You didn't list any other needs or priorities like battery life, performance, or even a budget, so I recommend going on any local used market app/site (Facebook marketplace, Craigslist, OfferUp, LetGo, etc.) and sorting phones by the cheapest available and buying the first option in that list.
Most carriers have a list of wifi calling "certified" phones. I'm not saying that something that's not on the list wouldn't work, but at least you have a starting point with your carrier.