In reply to a post by essex_man:

1) You have FTTP for A&A. Are they using CGNAT? If so, you may have a problem. It seems that the A&A Help page has some suggestions on how to overcome this.

AAISP is one of the most technically competant ISPs in the UK offering internet access and VoIP services. They have excellent support, especially if you get broadband and voice from them.

They do not CGNAT and every customer has v4 and v6 “proper internet”, and they are very critical of companies that don’t supply IPv6 to customers.

Edited by jchamier (Wed 29-Jan-25 22:25:39)

Hi,

This is almost certainly a firewall issue.

Fixing this will require you (or someone) to understand what range of RTP ports has been configured for VoIP and confirm that the firewall on the router has been configured to accept the same range.

I don't know whether the router will use IPv4 or v6 for VoIP and whether these are configured together or differently in the firewall - you may also need to understand that as well.

You get exactly the same symptom if NAT is blocking the incoming RTP stream (hence the existence of NAT ALGs). NAT will not be a problem here (unless the router has been configured in a very weird way to provide NAT to itself) as the router will have its own public IP address.