Configure P-Charge-Info for Private PBX (Example: FreePBX)

Learn how to add a P-Charge-Info SIP header for calls made through your private PBX (using FreePBX as an example). See Telnyx guidance and requirements.

Background

Some carriers and services, including Telnyx, use the P-Charge-Info SIP header to identify the billing number associated with a call. When using a private PBX like FreePBX, you can manually configure this header so that calls sent through your SIP trunk include the correct DID (billing number) in the SIP INVITE.

Standard Behavior

Calls are sent from your PBX to Telnyx via SIP trunk.
No P-Charge-Info header is included by default.

New Behavior (With P-Charge-Info)

PBX injects a P-Charge-Info header into SIP INVITE messages.
The header includes a specific DID from the Telnyx connection.
Telnyx uses this DID call identification.

Note: This configuration applies only when your PBX sends calls via IP-based SIP trunks. If multiple IP trunks are configured, apply the rule only to the relevant one.

Pre-requisites

Before you begin, make sure you have:

A Telnyx SIP Connection configured for outbound calling.
Access to your FreePBX Admin interface and the underlying server filesystem.
The DID number (E.164 format) that should appear in the P-Charge-Info header.
Root or admin privileges to edit FreePBX configuration files.

Step 1: Locate the Custom Extension Configuration File

FreePBX allows for custom dial plan extensions in:

/etc/asterisk/extensions_custom.conf

This file is used to extend default dial plan behavior safely, without affecting future system updates.

Step 2: Add the P-Charge-Info Configuration Block

Open the extensions_custom.conf file and add the following section at the bottom:

[macro-dialout-trunk-predial-hook] exten => s,1,NoOp(Entering user defined context [macro-dialout-trunk-predial-hook] in extensions_custom.conf) same => n,GoSub(func-set-sipheader,s,1(P-Charge-Info,+1541234567)) same => n,Verbose(2,Added P-Charge-Info: +1541234567) same => n,MacroExit()

Replace +1541234567 with your actual DID number from the Telnyx connection.

Tip: This macro ensures that every outbound call includes the P-Charge-Info header in the SIP INVITE.

Step 3: Apply and Reload Configuration

After saving your changes:

In FreePBX, go to Admin → CLI or SSH into your PBX.
Run the following command:

fwconsole reload

This reloads the Asterisk configuration without requiring a full restart.

Step 4: Verify the Header in SIP Messages

You can confirm that the P-Charge-Info header is being sent:

Enable SIP debug on the Asterisk console:
```
asterisk -rvv sip set debug on
```
Place an outbound call through the configured trunk.
Check the INVITE message for:
```
P-Charge-Info: +1541234567
```

Tips for Success

This assumes one IP-based trunk configured in the setup — the macro applies to all IP trunks by default. Update to target specific trunk if required.
Ensure the DID matches the number assigned to your Telnyx connection.
If calls fail after applying the macro, comment out the new lines and reload to troubleshoot.
Always back up extensions_custom.conf before editing.