Acer V360 - Manual (Page 58)

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Selecting which SIM to use when making a call
If you have set Voice call to Always ask, you will be asked which SIM to use each
time you make call. Enter the number, then tap . A dialog opens; tap the SIM to
use and the call will be connected.
Selecting which SIM to use when sending a text message
If you have set Messaging to Always ask, you will be asked which SIM to use each
time you send a text message. Enter the message and recipient information, then
tap Send. A dialog opens; tap the SIM to use and the message will be sent.