Sony Xperia 1 III Help Manual page 54

Hint
If you set one SIM card for calling and the other SIM card for data communications, you may not be able to access the internet
while talking on the phone. In this case, turn on the [Data during calls] switch of the SIM card for calling. You will now be able to
perform data communications even during calls.
If the Dual SIM reachability function does not work after you enable it, check that you have entered the phone numbers correctly
for each SIM card. In some cases, the numbers are detected automatically during setup. Otherwise, you are prompted to enter
them manually.
Related Topic
Settings menu
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