VTech VC7151 User Manual page 63

You can use your answering system and voicemail service together
by setting your answering system to answer before voicemail
service answers as described below. To learn how to program your
voicemail settings, contact your telephone service provider.
If you are on a call, or if the answering system is busy recording a
message and you receive another call, the second caller can leave a
voicemail message.
Set your answering system to answer calls at least two rings earlier
than your voicemail service is set to answer. For example, if your
voicemail service answers after six rings, set your answering system
to answer after four rings.
Some voicemail service providers may program the delay before
answering calls in seconds instead of rings. In this case, allow six
seconds per ring when determining the appropriate setting.
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