Call Cannot Be Established - 503 Service Unavailable

[Thiek]

3CX version 16.0.1078
Using a 23 Channel PRI Circuit
The device to bridge between 3CX and the PRI is a Patton SN4171 gateway

I'm wondering if anyone can provide more details about the following error message that we sporadically get, however, over the last few days many of our calls are not going thru. Sometimes I can all a number like 800-222-1000 which is AT&T and the call goes thru, I hang up and call it right back and get "call cannot be completed" ; the numbers we are dialing are valid so that's not the issue. I can see that there might only be one other person making an outbound call when I get the error, or I might be the only outbound call when this occurs - another words the number of outbound calls is not making a difference. I have the local PRI provider looking at the circuit and testing, they are helpful as to just closing out the ticket and saying our equipment is fine, and so they are the ones asking for more details on what generates this error. I did find in another post the below response to the 503 error, just wondering if anyone might have more insight that I could share with the PRI provider to help them resolve this issue?

Archie - 3CX Support: Actually, that 'Service Unavailable' response arises when PBX can not resolve some address. Or, possibly, your PSTN gateway sends this response to PBX by some reason

 

[leejor]

You , and your provider, may have to determine if there is a pattern to the failed calls...i.e, are they all to toll free numbers, the same numbers, certain area codes? You may also need to confirm (if you have not already done so) that the calls are actually being passed onto your provider, and not simply being "blocked" in the gateway.

 

[Thiek]

Thanks for the reply. No pattern, as mentioned I can call a number and get thru, hang up and call it right back and get the error. I have tried different area codes, my cell, my 3cx lab number, work desk, house, businesses, 800, 866, etc. and same thing where 2 or 3, or even 6 or 7 calls won't go thru and then finally it will go thru or 'connect'. I don't believe its hardware (the Patton device) or 3CX but solely with the local PRI provider or maybe long distance provider. Oh... and it is MUCH more likely to occur between 8am to 4pm on business days. To me its as if there isn't enough open circuits... sort of like going to a concern and waiting for a porta-potty to open. When Covid first hit and everyone (but us) was working from home I recall seeing these errors all day long from our different 3CX sites and many times around the top of the hour when conference calls would start. They didn't clear up until around June of 2020, however, every now and then at one of our sites a rash of these errors will occur for 3 or 4 days which is the case right now.

Back to Archie's response below - any more details about this error that I can provide to the phone tech?

Archie - 3CX Support: Actually, that 'Service Unavailable' response arises when PBX can not resolve some address. Or, possibly, your PSTN gateway sends this response to PBX by some reason

 

[leejor]

You may need to use Wireshark to see if calls are progressing beyond the gateway.Does the call get passed to the provider, then rejected, and that rejection is what is being passed back to 3CX from your gateway, or is the call simply being rejected by the gateway, not going beyond. You will need to determine that.

Actually, that 'Service Unavailable' response arises when PBX can not resolve some address.

I'm not sure about which address 3CX would be having an issue with, if you are using a gateway. That IP (of the gateway) should be static. It is not as if 3CX has to do a DNS lookup on a URL, at least, it shouldn't have to..

The second sentence, is, of course, a possibility. It very well may be an issue with a setting in the gateway.

I should have asked earlier...the gateway is on the same LAN as the 3CX server, correct?

 

[Thiek]

Yes, it's on the same gateway and has a static IP. I'll work with our Network team about the wireshark part.