The best replacement for asterisk is freeswitch.

The best alternatives to asterisk are rated as following:
★★★★★  freeswitch
★★★★  trixbox
★★★  3cx
★★★  freepbx
★★★  asterix
★★  avaya
★★  switchvox
★★  asterisknow
★★  elastix
★★  shoretel
★  pod
★  lync
★  sipxecs
★  asterisk
★  star
★  twilio
★  sipx
★  broadsoft
★  opensips
  mitel
  asterick
  callweaver
  pod mx
  openvbx
  evs
  call manager
  openser
  yate
  televantage
  avaya ip office
  podmx
  cisco callmanager
  footnote
  kamailio
  ampersand c++
  sipfoundry

Your alternative for asterisk is not listed? Suggest!
Speaking about asterisk, one probably shouldn't forget about skype.

Other search terms related to asterisk and their ratings are:
★★★★★ skype
★★★★★ freepbx
★★★★ vonage
★★★★ cisco phones
★★ google voice
★★ sip proxy
★★ sip
★★ lync
★ webrtc
★ trans
★ magicjack
★ ser
★ footnote
★ dialogic
★ broadvoice
gtalk
voip
lync 2013
sccp
openser
gsm gateway
cisco 9971
srtp
fwd
pbx
xmpp
You have in mind a companion for asterisk and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of asterisk in all years. Click on graph for more.
asterisk
freeswitch
trixbox