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The voice quality of your VoIP calls is determined by the codec used for the transmission. To increase the quality, more data must be transmitted. Depending on the bandwidth of your DSL connection, this can then lead to problems with the volume of data – especially if two VoIP calls are made simul taneously – so that the transmission no longer takes place smoothly. The following settings allow you to adjust your Gigaset to your individual DSL connection.
Settings -> Telephony -> Audio
You can set the following parameters for the voice quality for each connection:

Time interval for RTP packets

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Voice mail services

Some VoIP providers offer answer machines on the network – network mailboxes. These accept incoming calls on the corresponding VoIP phone number. 
You can access voice mails on the network mailbox within the VOICEMAIL area of the 
Image Added Call list app.
To record all calls, set up a network mailbox for each of your VoIP accounts. 
Settings -> Telephony -> Voicemail Services