Good question. We are trying to position the plug as a backup to a hosted solution as well as for residential environmnets. The costs for residential equipment after purchasing the plug one really needs to have an Analog Telephone Adpater (ATA) that they can purchase on ebay for around $60. This would be plugged into your home phone jack and provide dial tone for the house. You need to go outside and actually cut the cord coming from the phone network if you want to go that route since voltage is still coming down on the line. Linksys makes a PAP2 that is really popular and there are also FXS ports built into some of the Linksys routers as well. You can plug a cordless phone directly into the gateway if you didn't want to be disconnected from the PSTN. Next you need to get an ITSP like www.callcentric.com
who have pay as you go plans or even World select that includes unlimited calling to 40 countries and they can port your number over if want to keep it. The last step would be the pbxnsip license which costs between $50-$100 depending upon what features you need.