IP Telephony is VoIP (Voice Over Internet Protocol).
SIP (Session Initiation Protocol) is the VoIP protocol used by most VoIP adapters now days.
VoIP is free over the Internet, just like web browsing and email is free.
But VoIP needs a proxy server to setup the calls between the PC's or VoIP ATA's (analog telephone adapters). Just like you need a email server and web server to handle email and web, you need a VoIP server to facilitate the call setup of VoIP calls. The VoIP servers are needed to help traverse NAT Routers. Without the servers, you generally cannot get through the NATs. And, these days you don't want to be on the Internet without being behind a NAT Router.
There are many VoIP services like Skype, Google Talk, CallCentric, InPhonex that provide free VoIP calling over Internet.
Services like Skype and Google Talk work only with their proprietary softphones or firmware embedded phones.
Services like CallCentric and InPhonex are BYOD (bring your own device) services. Which means you can use any SIP-based user configurable (unlocked) Softphone or ATA VoIP phone adapter.
In-Network VoIP calling is always free with these services. Their servers only setup the calls between your VoiP devices, and then your calls are direct PC-PC or ATA-ATA.
You only pay money to make calls to traditional landline phones or cells. But, all In-network calls are completely free.
I configure my own Linksys SPA-2102-NA VoIP adapters to work with services like CallCentric, InPhonex, Les.net, Voip.ms. If both you and your family both have free calling accounts with CallCentric, then you can talk to each other over Internet for free with Softphones or ATA's.
The Linksys SPA-2102-NA has its own built-in NAT-router as well as ATA phone adapter. So, it connects directly to your Internet Modem. It also has excellent QOS for giving VoIP priority over other Internet data to your network through the ATA. I find it is an excellent user configurable ATA.