fpVoIP
From Free Pascal wiki
About
fpVoip is an attempt to create ambitious open source library in pure Object Pascal library to handle Voice Over IP (VoIP) Protocols such as SIP, IAX2, XMPP and also RTP, SDP and all other related protocols.
Also there will be implementation for codes such as ITU G7xx, BroadVoice, iLBC, SPEEX, GSM, CELT, Opus and more
Link
For more information you can find it at GitHub.