PicoPhone is a simple Internet phone application with chat. If you want a more advanced application, that has also video, file transfer, address book, that can call regular phones, send SMS messages to cellphones and more, take v-Phone.

I made Picophone, because all the Internet phones around are quite heavy and use complicated protocols, such as H.323, which has problems with NAPT routers. PicoPhone uses a simple UDP-based protocol, which works very well with NAPT. The program accepts connections on UDP port 11676 and makes connection to that port. Optionally a port number can follow the address (the colon is used as a separator), if the connection has to be made to another port (the NAPT router on the receiving side should convert the port number to the default 11676).

PicoPhone allows multiple concurrent calls, but the audio device should allow multiple output streams to be opened simultaneously.

On incoming calls the program plays the file ringin.wav, which should be present in the default windows location for wav files or in the same directory of PicoPhone.

Here is the version 1.65. It is free.
Here is the version 1.40, if somebody dislikes the new user interface. It is also free.

If you don't have a static IP address or if you are behind a NAPT router, the Picoserver service can be useful to you. Register here.

Important information: Picophone connects to www.vitez.it to obtain the list of picoservers and the public IP address of the computer, where Picophone is running. Its connection attempts are not spyware.

Las instrucciones están disponibles en Español AQUÍ (cortesía de Manuel Moguilevsky).
Instructions in French are available here (thanks to Etienne Neuville).
Russian users can go here.