We are in the process of adding SRTP/TLS encryption to sipgate trunking.
Until secure trunking is released, calls are transmitted without any encryption. A possible alternative would be to use encryption options provided via VPN.
If you are interested in testing our beta version of secure trunking, please give us a few details here.
NOTE: (15-08-2017) New registrations to beta test secure trunking are not currently being accepted.