The OBi supports single-line telephones only on the PHONE Port.
You can install a SIP client softphone such as X-Lite on your PC along with OBiAPP for PC and also make and receive calls through the OBi from your PC. OBiON Apps are also available for iPhone and Android smartphones.
It's also possible to interface an Analog Telephone Adapter (ATA), VoIP phone, or Asterisk with the OBi, but these options are somewhat more involved.