Nethvoice: Support inbound DID extraction from SIP header P-Preferred-Identity

[margit86]

Description

Some SIP providers use the P-Preferred-Identity header to carry the called number (DID) on incoming calls when the trunk is configured as a multi-number trunk.

In this scenario, the number normally used by NethVoice through the Asterisk configuration files to identify the inbound route does not allow the different DIDs associated with the same trunk to be distinguished.

For example, the provider sends an incoming INVITE containing:

P-Preferred-Identity: <sip:0123456789@provider.example>

where 0123456789 represents the DID that was called.

As a consequence, different inbound routes cannot be configured according to the number dialed by the caller.

Current behavior

At the moment this behavior can be implemented by manually adding a custom dialplan context to Asterisk's extensions_custom.conf.

The purpose of this feature is to include the required dialplan logic directly in the Asterisk configuration files provided by NethVoice, making the following context available by default:

from-pstn-preferredin

The administrator can then simply configure the trunk to use from-pstn-preferredin as its inbound context.

This avoids having to manually add and maintain the same custom dialplan code in extensions_custom.conf on every installation where a provider uses P-Preferred-Identity to transport the called DID.

The expected workflow would therefore be:

SIP trunk
   |
   | context = from-pstn-preferredin
   v
[from-pstn-preferredin]
   |
   | extract DID from P-Preferred-Identity
   v
[from-pstn]
   |
   v
standard inbound routing

Adding this context to the Asterisk configuration files provided by NethVoice makes the configuration reproducible and managed by the product, instead of relying on installation-specific changes to extensions_custom.conf.

Proposed solution

Add the from-pstn-preferredin dialplan context to the Asterisk configuration files provided by NethVoice.

When a trunk is configured with:

context=from-pstn-preferredin

the context should:

1. Read the P-Preferred-Identity header from the incoming SIP INVITE.
2. Extract the user/number part from the SIP URI.
3. Use the extracted number as the DID.
4. Forward the call to the standard from-pstn context.
5. Fall back to the original called extension if the channel type cannot be handled.

The feature remains explicitly enabled on a per-trunk basis simply by selecting from-pstn-preferredin as the trunk context.

Current workaround

The following custom dialplan is currently added manually to extensions_custom.conf:

[from-pstn-preferredin]
exten => _.,1,NoOp(Attempting to extract DID from SIP P-Preferred-Identity header)
exten => _.,n,GotoIf($["${CHANNEL(channeltype)}"="SIP"]?SIP)
exten => _.,n,GotoIf($["${CHANNEL(channeltype)}"="PJSIP"]?PJSIP)
exten => _.,n,NoOp(Unable to determine SIP channel type)
exten => _.,n,Goto(from-pstn,${EXTEN},1)

exten => _.,n(SIP),Goto(from-pstn,${CUT(CUT(SIP_HEADER(P-Preferred-Identity),@,1),:,2)},1)
exten => _.,n(PJSIP),Goto(from-pstn,${CUT(CUT(PJSIP_HEADER(read,P-Preferred-Identity),@,1),:,2)},1)

Once this context is included in the Asterisk configuration files provided by NethVoice, the administrator would only need to configure the incoming trunk with:

context=from-pstn-preferredin

without adding any custom dialplan to extensions_custom.conf.

Expected behavior

Given an incoming INVITE containing:

P-Preferred-Identity: <sip:0123456789@provider.example>

and a trunk configured with:

context=from-pstn-preferredin

the dialplan included in the Asterisk configuration files provided by NethVoice should extract:

0123456789

and continue the call through:

from-pstn,0123456789,1

allowing the standard inbound routing mechanism to differentiate the individual numbers belonging to the same multi-number trunk.