I cannot use data at the same time as making a voice call.

On networks that don't support VoLTE your handset will switch back to 3G to make a call. In this case you will not be able to use data while you are making a call. Many roaming networks don't support VoLTE.

1. Make sure VoLTE is enabled in your handset.

2. Make sure that 4G-Auto/Preferred or 5G-Auto/Preferred is selected.

3. Try to get a 4G or 5G network.