This is only possible if you have registered 2 VoIP accounts. The call quality will depend on your bandwidth, more specifically, your Up stream bandwidth.
The 2 VoIP accounts MUST be from the SAME VoIP Provider, and the individual account's settings must be entered in on the Voice, SIP page of the NB9 / NB9W.
Example:If you have registered for 1 VoIP account from Engin, and 1 account from GoTalk, you will not be able to use both accounts at once.
However, if you have registered for 2 VoIP accounts from Engin, you can use both accounts at the same time.
If you enter the same VoIP account settings into BOTH phone ports on the Voice, SIP page, then you will still be able to use both phones, although not simultaneously.