As a process of elimination can you try your sim card in another phone @FjR513
Also if possible try another sim card in your phone.
As mentioned by @cheezestrip this can also be your network.
Does this happen everywhere or in one place?
If your Network has an online mast checker then see if any issues are being reported.
Also sometimes a sim card fails to which your network will be able to sim swap to replace it.
I wish you all the best with this situation.
🌈 Stay Safe & Stay Alert. 🌈