IPhone SE 2020 unable to make or receive calls

[LT-2020]

My Iphone SE is unable to make or recieve calls. When I call anyone, I hear three beeps and then the call disconnects. There is no error message of any kind. When someone calls me it says they have dialled an incorrect number.

I have tried turning it on and off several times, removing the SIM card, resetting network settings, checking for network problems in my area etc. 

I have full signal bars and I am able to send and recieve texts.

I upgraded my phone a few days ago and completed the sim swap process. I didn't have this problem before upgrading/swapping sim. 

Does anyone have any ideas? I have tried speaking to someone on the Online Chat but after going round in circles for an hour they referred my case to a different department which may take 5-7 days to get back to me.

[MI5]

@LT-2020 

Sounds like the sim swap may have failed.

Can you check your old sim in another phone to see if that is working?

The other possibility is that your phone is barred, so try the iPhone sim in another phone too.

Let us know the results of these tests first please.

[LT-2020]

Thanks for your help!

I tried the old sim and it doesnt work, has no service etc.

I then tried the new sim in a different phone, still the same problem with calls. 

[MI5]

@LT-2020 

Faulty new sim at a guess.

Can you get to an O2 store?

They will swap it for free whilst you wait.

[LT-2020]

O2 sent me a new SIM which arrived today, however before I did the swap, someone called me and I was able to make calls after that. Whatever the problem was it managed to fix itself!

[MI5]

@LT-2020 

Keep the new sim as a spare, just in case you need it in the future.