@pzanga
My provider used to source RTP directly from the carrier servers (not their own) but always had their own SIP.. (has since changed)
Depending on what you have on your end.. Server or Clients.. Look to see what your clients are connecting to. If you have a local on site SIP server it will still act as a client to go out to the providers server generally.
Look at connection states to see what things are trying to connect to. Do you have DNS of their stuff? sip.frontier.com?? ect..??
Outbound everything is allowed by default. In some cases the provider does want inbound NAT (I don't usually agree with) and in some cases you will only need firewall rules to allow the traffic to the client.
Maybe the Siproxd package is a good fit for you?