Aastra M401 User Manual page 49

If another party tries to call you, you will hear a beep, and the light on a second
programmable key will start flashing.
To take the second call:
Press the second programmable key
The key light will stop flashing and remain on.
The first party is placed on hold, and the first key light will start flashing again.
You are in conversation mode with the second party.
You can switch from one party to the other by pressing the respective keys.
Special features
PROGRAMMABLE KEY 2
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