Huawei honor8Pro Guide De Démarrage Rapide page 7

Dual card dual standby single pass
Your phone supports dual SIM cards. Both SIM cards are
available at the same time when you are not using your
phone. However, while one SIM card is active, the other is
unavailable. This is known as dual card dual standby single
pass.
For example, while you are using SIM card 1's data service, SIM
card 2's data service will be disabled. And, while you are on a
call using SIM card 1, SIM card 2 will be unavailable. Someone
calling SIM card 2 while you are on a call using SIM card 1 will
either reach your voicemail or hear a message such as "The
person you called cannot be reached at the moment. Please try
later." The message heard will vary depending on service
provider.
Your phone's single pass feature helps reduce power
consumption and increase standby time.
Contact your service provider for information about subscribing
to notification services.
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