What is P-Asserted-Identity?
P-Asserted-Identity (PAI) is a SIP header used to convey the verified identity of the caller — typically the phone number — between trusted network elements such as SIP trunking providers and carrier networks. Unlike the From header, which can be freely set by end-user equipment, the PAI header is intended to carry a trusted, network-verified identity.
Carriers rely on PAI to determine the true originating number for purposes such as caller ID presentation, call tracing, and regulatory compliance.
How the Atomic Softswitch Handles PAI
The Atomic Softswitch fully supports the P-Asserted-Identity header on all outbound SIP INVITE messages. Here is what you need to know:
When You Send a PAI Header
If your equipment (PBX, softswitch, or SBC) includes a PAI header in the SIP INVITE, the Atomic Softswitch will pass it through to the carrier as-is. The From header is not modified.
To maximise compatibility across both domestic and international carrier networks, we strongly recommend formatting your PAI number in E.164 format (e.g. +61412345678). Domestic formats and international formats without the leading + may work in some cases — see the format guide below — but E.164 is the safest choice.
When You Don't Send a PAI Header
If your INVITE does not include a PAI header, the Atomic Softswitch will automatically generate one using the CLI (Calling Line Identity) you have passed in the From or Contact header. This ensures your calls are not rejected by downstream carrier networks that require a PAI header.
Multiple PAI Headers
Some PBX systems send multiple PAI headers in a single INVITE. The Atomic Softswitch supports this — all PAI headers will be passed through to the carrier. Keep in mind that the carrier will determine which value to use, and behaviour may vary between carriers.
Accepted PAI Number Formats
For domestic Australian calls, the following number formats in the PAI header are supported:
Format | Example | Notes |
|---|---|---|
| E.164 (Recommended) | +61412345678 | Best compatibility. Required for international calls. |
| International | 61412345678 | Without leading +. Works on most domestic carriers. |
| Domestic | 0412345678 | 10-digit local format. Accepted for domestic calls. |
Note: If the PAI header is sent in an unsupported or malformed format, the Atomic Softswitch will pass it through as-is. However, the downstream carrier may reject the call. Always perform test if using non-E.164 formats.
Common Scenarios
Here are a few real-world examples to help illustrate how PAI is handled depending on your setup.
| Description | Your PBX sends a SIP INVITE with a correctly formatted PAI header using the international E.164 format. |
|---|---|
| PAI Header | P-Asserted-Identity: <sip:+61298765432@yourdomain.com> |
| Result | The Atomic Softswitch passes the PAI header through unchanged. The carrier receives and presents the number as-is. This is the ideal setup. |
| Description | Your PBX sends the PAI header using a 10-digit domestic number format rather than E.164. |
|---|---|
| PAI Header | P-Asserted-Identity: <sip:0298765432@yourdomain.com> |
| Result | The switch passes the header through as-is. For domestic Australian calls, this will generally work. However, for international or some carrier interconnects, you may experience compatibility issues. We recommend switching to E.164 format to be safe. |
| Description | Your PBX or SBC does not include a PAI header in the SIP INVITE at all. |
|---|---|
| PAI Header | (no PAI header present in INVITE) |
| Result | The Atomic Softswitch automatically generates a PAI header using the CLI (number) from your From header. For example, if your From header contains 0298765432, the switch will create: P-Asserted-Identity: <sip:0298765432@switch.atomtelecom.com.au>. The call proceeds normally. |
| Description | Your PBX or SBC sends more than one PAI header in the same SIP INVITE — this is common with some enterprise PBX systems. |
|---|---|
| PAI Header | P-Asserted-Identity: <sip:+61298765432@yourdomain.com> P-Asserted-Identity: <sip:+61412345678@yourdomain.com> |
| Result | The Atomic Softswitch passes all PAI headers through to the carrier. The carrier will apply its own logic to determine which value to use for caller ID presentation. Behaviour may vary between carriers. |
| Description | Your PAI header contains a number in an unrecognised format, or is missing required SIP URI formatting. |
|---|---|
| PAI Header | P-Asserted-Identity: <sip:98765432@yourdomain.com> |
| Result | The Atomic Softswitch will pass the header through as-is without modification. The downstream carrier may reject the call or strip the header. If you are experiencing call failures or missing caller ID, check that your PAI header is correctly formatted. |
Anonymous / Private Calls
If you need to make anonymous or private calls, please note that special handling is required for the PAI header. Refer to our separate Knowledge Base article: Anonymous / Private Calling.