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 TD-8816, TD-8817, TD-8840T, TD-8841T, TD-W8101G or TD-W8901G modem you may experience problems such as voice being 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 to 192.168.1.1.
ip nat service sip sw off
If the command is successful the following line will be returned:
nat sip v2 switch off
Note that this configuration change cannot be saved and so when the modem is rebooted or powered off, the command will have to be re-run.
Linux and Mac
On Linux operating systems you can access telnet by opening a terminal and typing in "telnet 192.168.1.1" at the prompt.
On Windows XP and Vista operating systems telnet can be accessed by opening "run" and the "cmd" command and entering telnet.
In Windows 7 and 8 you can bring up the command line by entering "cmd" in the search bar.
In any version of Windows you can also run telnet by installing and using the PuTTY free software application.
For complete Windows instructions see this TP-Link page.