Hello there.
Welcome to the Three Community.
There's a few things worth looking at/considering. The first is Wifi calling. This is designed to offload network services onto Wifi whenever it benefits the user. In some cases it can cause issues though. Once way to rule it out is to turn off Wifi calling, and then see if you still have the issue. Another way to test would be to go into Airplane mode, then ensure your Wifi is on, this will force Wifi calling to be active and test again. If your issue goes away during the first test, it could point to your Wifi connection not being ideal for Wifi calling features, and the feature should be left off. If the issues go away during the 2nd test with Wifi calling being the only connection, it could point to an issue on the mobile network side of things.
To check out the mobile network side of things, you can use the Service Status Page, which will give you an indication of how things are in the area.
If non of these give answers, you may have a device issue. You may want to try resetting the network settings within the phones reset menu. You could also look at replacing your SIM card. If your phone supports eSIM, you can do the swap quickly without any down time and without having to go to store to store.
Pete.
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