The best replacement for pstn is voip.

The best alternatives to pstn are rated as following:
★★★★★  voip
★★★★★  isdn
★★★  pots
★★★  pbx
★  voip security
★  plmn
★  ngn
★  pri
  pabx

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

Other search terms related to pstn and their ratings are:
★★★★★ asterisk
★★★ trixbox
You have in mind a companion for pstn and it is not yet here? Propose!



Google Trends

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