VOIP or Voice over IP, is a term designed to refer to a set of methods and technologies targeted for the implementation of telephony services over the Internet. If you wish to gather more information about VOIP itself, there is a website that labels itself “A reference guide to all things VOIP” and it holds comprehensive information on the matter.
SIP is the Session Initiation Protocol – a protocol used for signaling and handling communication sessions. This protocol is sometimes referred to as the de facto standard for VOIP implementations. It is an IETF (Internet Engineering Task Force) standard even though there are other options to SIP, such as IAX2. SIP employs the RTP protocol for data transmission which itself is encapsulated in TCP or UDP and can be encrypted by using TLS.
A company named ABTO Software provides the ABTO VoIP SIP SDK, which has a version for Windows CE named ABTO VoIP SIP SDK for Windows CE. One of our customers has reported using this tool successfully.
There is a blog post from July-2016 published in CNX Software that talks about Setting a VoIP SIP user agent with Embedded Linux. The PJSIP libraries and the high level PJSUA API were employed.
You can also try to search for SIP libraries and softphone among OpenEmbedded recipes. At the time of writing, there were a few options not tested by Toradex, such as the Linphone softphone and the Sofia-SIP library.