You will need to reach out to Voxtelesys and schedule a time to move your trunk to TLS/SRTP, this feature will be coming to the portal soon but for now it requires a ticket with email@example.com
Open your Voxtelesys SIP trunk and ensure that your Registrar/Server/Gateway Hostname or IP is set to 3cx.pstn.voxtelesys.net
If you have a unique domain name like xxxxxx-xxxx.sip.voxtelesys.net then reach out to support and request a new registration domain.
Go to Options and change the following settings:
SRTP Mode = Enabled
Transport Protocol = TLS
Before saving the trunk settings we will need to upload the root certificate for the domain.
Go to https://letsencrypt.org/certificates and download the Active "PEM" certificate.
Go back to the 3CX and upload the certificate under options > Transport Protocol = TLS > Click "Upload
You should now see an expiration date.
Save the Trunk settings
Let voxtelesys support know that the TLS/SRTP is in place on the 3CX trunk and support will verify the correct settings are enabled on your trunks!