Turn on Wi-Fi Calling so you can make and receive calls over a wireless network when cellular signal is weak.
Tap the Settings app, then tap Cellular (called Mobile Service or Mobile Data in some regions). If you have two SIMs, choose the line you want to enable.
Inside the Cellular menu, scroll to and tap Wi-Fi Calling. Not all carriers support it; if you don't see the option, contact your carrier.
Tap the switch so it turns green, then tap Enable on the confirmation prompt. You may be asked to confirm or enter your emergency address, which is used to route 911 calls when on Wi-Fi.
Connect to a Wi-Fi network and check the status bar — it will show your carrier name followed by Wi-Fi (e.g., Carrier Wi-Fi). Place a test call to confirm it works.