How to avoid roaming question on dual-sim calls?
In my Android 4.2.1, I have to set Settings &rarr; SIM Management &rarr; Roaming &rarr; Show roaming reminder &rarr; Never.

Alternatively, use adb to disable the notification for a particular SIM. 

Setup ADB in PC, enable USB debugging in device, connect the device into PC, launch a shell on latter and follow these instructions:

To enable data roaming for default SIM, enter:         

adb shell settings put global data_roaming 1


Changes would take effect immediately. For SIM2, change the key to data_roaming_2.

If you want to know the details about this command, enter adb shell settings. You would easily come to know what the aforesaid command would do.