The client does a ZRTP handshake with a media relay which sets up the PSTN call. We are adding POPs depending on traffic in order to get that ZRTP termination as close as we can to common destinations, ideally minimizing as many border crossings as possible (as that is, at least theoretically, where the greatest risk of interception exists).
The goal of PSTN calling isn't to provide bulletproof security. The goal is to provide a somewhat reduced exposure, and to allow you to use the app as your default dialer, hopefully making it a habit. If you need your conversation to be very secure, then both parties should be using the app (and hopefully you're both in the habit of using it anyway, since you can make your PSTN calls there as well).
The goal of PSTN calling isn't to provide bulletproof security. The goal is to provide a somewhat reduced exposure, and to allow you to use the app as your default dialer, hopefully making it a habit. If you need your conversation to be very secure, then both parties should be using the app (and hopefully you're both in the habit of using it anyway, since you can make your PSTN calls there as well).