On the first page, there's the claim:

  Encrypted Calls
  
  All calls are encrypted with TLS 1.2+ for signaling and SRTP with AES-128 for audio. Your conversations stay private during transmission.

This can only be true on your (Voklit) end between the client and the server, it stops being encrypted both from signalling and audio perspective the moment it hits the PSTN in any country on any network.

I would consider re-phrasing this so that limitation is more clear, for people who may misinterpret that encryption claim, unless you've found a way to encrypt PSTN signalling and audio which would be a much more impressive feat.