this is an update to my previous entry, and does explain the architecture and all the steps you’ll need to set it up.
We make use of the pptp capabilities of iOS and build an pptp tunnel to a fixed server @.
From there we have an openvpn tunnel to our asterisk box *, which has ISDN links to the PSTN.
Routing makes shure, that all SIP traffic is shoveled between the endpoints.
Voilà encrypted calls, with your favourite SIP app.