VOIP through an ATA on 5G home broadband with CGNAT

Hi all,

I live in a very rural property with awfully slow BT broadband, which I am planning to ditch for 5G. I've bought a proper 5G router and tested it on EE (or rather 1Pmobile, an EE MVNO) and it's 20x faster than BT and seems reliable over a month.
I would now like to cancel my BT phone line but retain my landline number and transfer onto VOIP - I'm going to use Andrews&Arnold's £1.20/month VOIP service, with a Grandstream HandyTone HT801 ATA.

My question: does anyone know whether the HT801 will object to being behind double-NAT? EE use carrier-grade NAT (which I can't avoid) and then my 5G router is obviously NATing again. So I'll have no way to open ports to the "real" internet, should the ATA need them.

Is this going to cause problems?

Thanks

Most likely, NAT will cause some problems to VOIP (and double NAT may make this worse). There are ways around that using a STUN server. Outbound calls are more likely to work than inbound.

I am using VoIP (from A&A) over 4G for almost three years now, but this is on Three who will assign public IP on their 3internet APN. I have not dared trying a CGNAT APN.

Apparently, EE supports IPv6 which doesn't use NAT. So if the Grandstream, your 5G router and A&A support IPv6, then that would be the way to go. Failing that, you can always get a VPN that gives you a public IP address, but then you would need a device to terminate the VPN at your side (unlikely that the Grandstream or your router will support that).

Or you could move to Three if their coverage is any good in your area (might be worth testing with a cheap PAYG SIM).

I tend not to use ATA’s - well I did many moons ago, with a different setup, but they're not my favourite beasts. These days I mostly use a very decent VoIP soft client on my mobile - which has no problem traversing all manner of network ills, whether connected over the mobile carriers network or WiFi.

Suggest that you get a temporary VOIP number from someone like Voipfone and do some testing variously when behind your 5G router…

https://www.voipfone.co.uk/plans/trial-residential

I use Voipfone through a HT801 ATA, NAT and CGNAT with no problems. So it can be done (I claim no credit for that - I got the ATA pre-configured from Voipfone). But others with a similar setup do seem to have issues. Seems to depend a bit on both the competence of the Voip provider to do NAT traversal properly, and the stability the "real" IP address you get on the other side of the your ISP's CGNAT, which varies with ISP. The Voipfone config doesn't seem to use STUN, BTW.

I agree that ATA boxes aren't the ideal solution. In my case it was intended as a transitional thing - I'm trying to phase out the landline and just use mobile, but I've had the landline number for nigh on 40 years. Trying to find all the elderly friends who have it written down somewhere, tell them my mobile number, and then getting them actually to use it in preference is proving somewhat of a challenge. The web management interface on the HT801 is Neanderthal by today's standards, but all the parameters you could possibly want are in there somewhere. And it just works, so I rarely need to look at it.

Edited by Thaumaturge (Thu 05-Dec-24 14:44:37)