If you have dual SIMs, you can select one to be a default line for calls and data. It also doesn’t have to be the same line. You can specify a default for each one.
Calls and SMS
To set REALLY as the default calling line on iOS:
From your phone’s Home screen, select Settings
Scroll down and select Cellular
Tap Default Voice Line and select your REALLY phone number (It’s selected when a checkmark is present)
Data
To set REALLY as the default data line on iOS:
From your phone’s Home screen, select Settings
Scroll down and select Cellular
Click on Cellular Data
Tap Default Voice Line and select your REALLY phone number (It’s selected when a checkmark is present)
Cellular data switching
Turning the feature “Allow cellular data switching” on will allow your phone to use cellular data from either line, depending on best coverage and availability.
Note: With each iOS software update, these steps might change. The change should be minimal but please keep that into account!