The best replacement for sip is pri.

The best alternatives to sip are rated as following:
★★★★★  pri
★★★★★  voip
★★★  iax
★★★  icf
★★★  sccp
★★★  h.323
★★★  h 323
★★  mgcp
★★  skype
★★  rtp
★  tdm
★  h323
★  gulp
★  shoot sake
★  xmpp
★  ip
★  smtp
★  chug protein shake
  pbx
  spray foam
  soc
  chug water
  webrtc
  h 248
  skinny
  swig
  h.248
  uma
  mip
  sip t
  ppf
  rd which is better
  mpls
  qsig
  direct feed
  ss7
  sigtran
  isdn

Your alternative for sip is not listed? Suggest!
Speaking about sip, one probably shouldn't forget about google voice.

Other search terms related to sip and their ratings are:
★★★★★ google voice
★★★ tls
★★★ sdp
★ rtp
★ windows messenger
stun
asterisk
voip
You have in mind a companion for sip and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of sip in all years. Click on graph for more.
sip
pri
voip