new-ish iPhone often won’t make or receive calls or texts although data works

To make it clear, the phone shows it is connected to 4g and it is, in the sense that internet, WhatsApp, google maps etc all work, but i cannot make calls or send texts, text messages don’t arrive and incoming calls silently drop to voicemail. 

When I notice this has happened, I can usually fix it by turning off wifi, turning airplane mode on then off, although I may have to do this a couple of times.

I Rarely send trxts, but I have had frustrated callers complaining I never answer my phone and it is very difficult to receive 2fa text messages soon enough that I don’t have to restart the login process. 

Please help.