WiFi won’t work while on a phone call

Is it really WiFi that isn't working? Or, is it Cellular Data that stops working when you are on a phone call?

Are you a Verizon or Sprint subscriber? Are you in a remote location with barely any signal? If so, you may be only getting a 2G CDMA signal, which will exhibit the behavior you describe.

Could this be your problem?

You would have to know if you are using wi-fi calling, as that is not a default setting on the phone and requires you to manually turn it on. There should be nothing that would prevent you from using wi-fi when on the phone, as it uses two separate things.

If wi-fi assist is turned on, it would make me believe that your wi-fi is not operating correctly, and the device is trying to use cellular for data, and your provider does not support that. If you are not in a phone call and try to use your iPhone to go to a website in Safari while connected to wi-fi, can you do it? If you were to go into Settings and turn off cellular data and try to connect to a different website, can you do that? If you cannot access the internet while on wi-fi only, then there is an issue with your wi-fi.

Have you tried using wi-fi without Wi-Fi assist being turned on? The only other thing I can recommend that you do is to restore the device to factory and set it up as new and see if it works. I am not aware of any reason for wi-fi to not work when you are on the phone. If you want, contact Apple Support before restoring the phone and see what they have to say.

Do you see your wi-fi fan to show a signal for wi-fi? If you go to Settings>Cellular, scroll down and is Wi-Fi Assist turned on? Are you using Wi-Fi calling when making a phone call?