Troubleshooting Tip: How to change the 'SIP PAI User Part' field on SIP-INVITE message

SIP-Trunk was configured as follows:

config trunk sip-peer
    edit Liberty
        set status enable
            config did
            ...
            end
        unset number-pattern
        unset caller-name
        unset caller-number
        set sip-server 172.31.255.254
        set dns-srv-record disable
        set sip-port 5060
        set incoming-call-src-port-match 0
        set auth-incoming disable
        set username 7878573500
        set password ENC
        set auth-name 7878573500
        set registration standard
        set proxy disable
        set realm 172.31.255.254
        set sip-setting CLARO
        set callerid-pai-header specified
        set specific-pai-header 7878573500 <-- This info was added into the 'SIP PAI Display Info:', but not into the 'SIP PAI User Part'.
        set diversion-header-use on-redirect
        set diversion-header-source did-mapping
    next
end

However, on a packet-capture the field 'SIP PAI User Part' does not display the information required by the SIP-Provider.

Internet Protocol Version 4, Src: 172.31.255.250, Dst: 172.31.255.254 User Datagram Protocol, Src Port: 5060, Dst Port: 5060 Session Initiation Protocol (INVITE)     Request-Line: INVITE sip:7873824501@172.31.255.254:5060 SIP/2.0     Message Header         Via: SIP/2.0/UDP 172.31.255.250:5060;rport;branch=z9hG4bKPj2edf5a37-7167-441b-bdc9-68fd619bc9a4         From: "Juan Carlos Ríos" <sip:7879470204@172.31.255.254>;tag=30e1f179-233e-4fc6-8cef-96377e855683         To: <sip:7873824501@172.31.255.254>         Contact: <sip:7878573500@172.31.255.250:5060>         Call-ID: 2ac8d9a5-a59a-4f98-a4c7-a13c90628bcc         [Generated Call-ID: 2ac8d9a5-a59a-4f98-a4c7-a13c90628bcc]         CSeq: 5030 INVITE         Allow: OPTIONS, REGISTER, SUBSCRIBE, NOTIFY, PUBLISH, INVITE, ACK, BYE, CANCEL, UPDATE, PRACK, MESSAGE, INFO, REFER         Supported: 100rel, replaces, norefersub, histinfo         P-Asserted-Identity: "7878573500" <sip:SIP PAI User Part@172.31.255.254>             SIP PAI display info: "7878573500"             SIP PAI Address: sip:7879470204@172.31.255.254                 SIP PAI User Part: 7879470204                 SIP PAI Host Part: 172.31.255.254         X-FV-TRUNK-NAME: trunk_Liberty         Max-Forwards: 70         User-Agent: FortiVoice-500F         Content-Type: application/sdp         Content-Length:   415     Message Body Session Initiation Protocol (SIP as raw text)

The next configuration change must be applied to solve the requirement of the SIP-Provider:

config trunk sip-peer
    edit Liberty
        set specific-pai-header 7878573500<7878573500>
    next
end