3CX - Tips and Tricks


This guide is our current go-to for best practises while using Skyetel with 3CX.

You Need Good DNS Servers

SRV records are advanced technologies and rely on strong DNS resolvers. Please make sure you specify a well known and reputable public DNS server explicitly on your PBX to insure DNS resolution does not fail. We recommend and

3CX Routes on E.164 Format

By default, Skyetel routes all calls to the PBX using 11 digits. We do this in the SIP URI (the first line in the SIP INVITE packet). For example, when you call our number, this is what the SIP URI looks like:

INVITE sip:13609865200@ SIP/2.0

However, in order to organize our traffic, we send all Caller ID information in E.164 format. Therefore, the TO in a Skyetel INVITE looks like this:

To: <sip:+13609865200@>

This discrepancy confuses 3CX - it would prefer that the To and the URI are identical.

As part of our interconnection, 3CX is configured to route using the TO in E.164 instead of 11 digits. This means you should add phone numbers to 3CX like this: +13609865200

However - in our legacy 3CX guide, we recommended using a wildcard:

In order to accommodate this, we recommend you use a wildcard for your Skyetel numbers. Per 3CX documentation:

3CX will match the number inserted in this field with the “to” header, starting from the last part of the received string, thus avoiding any differences in the format of the number. For example, if you are based in the UK and your DID number is 0845-2304024, then you can enter the number *2304024. This will match any DID number inserted in the “to” field ending with these numbers, including +448452304024, 08452304024, 00448452304024, and, of course, *2304024.

This means that we would recommend that you configure your DIDs in this format: *3609865200

This method is still supported and will work correctly. However, you will not be able to route using 11 digits using the certified interconnection with 3CX.

Skyetel was certified on version 16 and 15

Skyetel was tested and certified with 3CX using version 15 and 16 and will be certified for all future version. Previous versions are not supported and you will need to use our Legacy 3CX configuration in order to get Skyetel to work on those PBXs.

3CX.skyetel.com Uses Port "0"

This is normal - 3cx.skyetel.com is an SRV DNS record that is purpose built for 3CX (if you are not using 3CX, please use srv.skyetel.com instead). SRV records contain the port and protocol information in the SRV lookup:

id 13608
opcode QUERY
flags QR RD RA
_sip._udp.srv.skyetel.com. IN SRV
_sip._udp.srv.skyetel.com. 299 IN SRV 10 10 5060 va1.skyetel.com.
_sip._udp.srv.skyetel.com. 299 IN SRV 10 10 5060 ca1.skyetel.com.
_sip._udp.srv.skyetel.com. 299 IN SRV 10 10 5060 or1.skyetel.com.

Because SRV records contain all of the relevant information about port, priority and source IPs, you can safely trust in 3cx.skyetel.com or srv.skyetel.com

Don't Add anything to the Outbound Proxy

When you are troubleshooting issues, make sure you leave "Outbound Proxy" blank (smile)


Keep Your Phone Numbers Unique

The most common issue 3CX has when receiving calls from our network is that Administrators forget to keep their Trunk phone numbers unique per trunk. Duplicate phone numbers across multiple trunks will cause 3CX to reject any attempt for us to send inbound calls. 

Main Trunk Number

You can use any number you would like for this, but please make sure that it is unique per trunk.

408 Request Timeout

On occasion, 3CX will fail to place outbound calls and give this error:

SIP Server/Call Manager ID: 12294​
Call or Registration to ###########@(Ln.10000@Skyetel) has failed. replied: 408 Request Timeout; internal​

This is caused by the TCP protocol timing out when communicating with our routers. This is not a 3CX bug, but it is a bug with some operating system's underlying networking stack. In order to resolve this issue, simply switch from TCP to UDP in the SIP Trunk Options. You can read more about this issue on the 3CX forums here: https://www.3cx.com/community/threads/3cx-dns-srv-resolution-issues.67708/

Was this article helpful?
2 out of 2 found this helpful
Return to top