VOIP means Voice Over Internet Protocol, where your telephone service is provided via the internet.
Using VOIP with a TP-Link modem
If you are using VOIP with a TP-Link modem you may experience problems such as voice is received but not transmitted after a connection is established. This may be addressed with a command line setting change in the modem to enable this via telnet.
ip nat service sip sw off
If the command is successful the following line will be returned:
nat sip v2 switch off
On Linux operating systems you can access telnet by opening a terminal and typing in "telnet" at the prompt.
On Windows XP and Vista operating systems telnet can be accesses by using the run command and entering telnet.
Windows 7 and 8 does not support telnet out of the box, but is can be run by installing and using the PuTTY free software application.