I’d love to know about this setup! What hardware you run it on and where you get the #’s from for the home line. Trying to set up something like this for myself at home and business. Thank you!!
Yealink T23P Phones have a OpenVPN client built in (some newer Grandstream phones do too), these are relatively inexpensive VoIP desk phones. Once you configure them, and an OpenVPN server you can plug them anywhere you have internet (NAT'ed or otherwise), and not have to worry about NAT Traversal, or other issues.
Asterisk is just running on a stock PC. I'm running on a Supermicro Board with an Atom processor, and for our call volume (50 extension phones, 23 inbound lines, 30 on an inbound call queue, 3 simultaneous calls average, 15 @ our known peak, no transcoding (all uLaw)). The OpenVPN server is a a separate machine, for lighter usage the VPN + Asterisk could probably be the same machine.
For the cell phone, I have CSipSimple running there, with a OpenVPN client to connect to the network.
In my configuration, the phones + Asterisk are not using SSL/TLS directly, but the VPN secures the traffic over untrusted LANs.
Calls within the system are on the protected LAN, but once they reach out to the PSTN, all bets are off.
I've setup a smaller office with a Raspberry Pi 3, and a Grandstream SIP/Analog Gateway, ( 7 Grandstream phones, 3 lines, no transcoding, not very heavy use), and they haven't had any complaints. (see http://www.raspberry-asterisk.org/) If they used SIP trunking over their internet connection, they woulden't need the SIP/Analog gateway, which was the single most expensive piece of equipment in this setup (@ US$399-ish)