Huawei P9 Lite - Manual (Page 79)

Calls and Contacts
72
Answering a third-party call
Before using this feature, check that you have subscribed to a call hold service and
enabled call waiting on your phone. For more information on enabling call waiting,
see Enabling call waiting . For details, contact your carrier.
1
If you receive an incoming call when in another call, touch . You can then answer
the incoming call and hold the other line.
2 Touch to switch between the two calls.