Snom 220 - Manual (Page 31)

Advanced Functions
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When the other side is available, the display will prompt you to dial the
number again.
DTMF Tones
During a call, e.g. with a voicemail system, pressing the digits 0-9, *, or
# will generate and send DTMF tones to the other party.
Short Messages (SMS)
Incoming short messages are automatically displayed when the phone is
in idle state.
Key mapping:
Scrolls the message forward.
Scrolls the message backward.
ENTER
Shows the next message or the idle
screen.
Short messages can be received from clients like Microsoft Messenger,
etc.
Message Waiting Indication (MWI)
If you did not answer a call and the mailbox recorded a message for
you, the idle screen shows that a recorded message is waiting for you.