1) Your phone needs to have Wi Fi calling feature. These days even the cheap $200 phones have such feature.
2) You NEED to enable Wi Fi calling with your provider. Inside your account there should be the option to Enable Wi Fi calling.
My phone is just a cheap Motorola Android, almost 4 years old now. The carrier is also the cheap Mobile Virtual Network Operator, MVNO. Mint Mobile. It uses T-Mobile network. Last year T-Mobile acquired Mint, much like Verizon acquired Trackfon. It has Wi Fi Calling feature. I just activated it when I purchased the plan. Mint is a prepaid plan operator with probably the cheapest yet high quality service among its peers. The best thing I like is the VERY CHEAP International calling rates, and of course, the Wi Fi calling feature. I have NOT bought a single telephone card for all our trips which probably far outnumber 90% of the posters on this forum except a few famous IDs. On top of that our trips usually are one month or longer. Leaving this Saturday for a 42 days long trip for example. Yet, I have not bought a telephone card. The WiFi calling / texting features are more than what I need. Plus these days many BnB hosts / Tour guides use WhatsApp as their main communication tool. You only need WiFi to use WhatsApp and it is free. You can send / receive pictures / videos, make voice / video calls. Unless you really really need to use Data when there is no WiFi around, then yeah, you need a data card.
Below explains what is MVNO
https://en.wikipedia.org/wiki/Mobile_virtual_network_operator
Below is Android phones Wi Fi calling function
Turn on Wi-Fi calling
https://support.google.com/phoneapp/answer/2811843?hl=en
Apple's iPhones have the same feature you just need to google it.
