Strange SIP register with false SIP Invite recall

I have here a smartphone app (DIVUS VIDEOPHONE app for iOS version 4.2.16) which makes a SIP REGISTER on an AVM Fritz!Box 7490 and then later should get a SIP INVITE. I just do not know... Does the app now make a wrong REGSITER or does the Fritz!Box make a wrong INVITE? What is of course clear that 192.0.0.1 is not accessible over the WAN.

SIP-REGISTER:

Internet Protocol Version 4, Src: 80.187.119.148, Dst: 79.237.225.35
    Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
    Total Length: 771
    Identification: 0x5448 (21576)
    Flags: 0x4000, Don't fragment
    Fragment offset: 0
    Time to live: 52
    Protocol: UDP (17)
    Header checksum: 0xf641 [validation disabled]
    Source: 80.187.119.148
    Destination: 79.237.225.35
User Datagram Protocol, Src Port: 27994, Dst Port: 5060
    Source Port: 27994
    Destination Port: 5060
    Length: 751
    Checksum: 0xf75e [unverified]
Session Initiation Protocol (REGISTER)
    Request-Line: REGISTER sip:foo.bar.net SIP/2.0
        Method: REGISTER
        Request-URI: sip:foo.bar.net
            Request-URI Host Part: foo.bar.net
        [Resent Packet: False]
    Message Header
        Via: SIP/2.0/UDP 192.0.0.1:10573;branch=z9hG4bK-524287-1---8db3ab2e48be0b2c;rport
            Transport: UDP
            Sent-by Address: 192.0.0.1
            Sent-by port: 10573
            Branch: z9hG4bK-524287-1---8db3ab2e48be0b2c
            RPort: rport
        Max-Forwards: 70
        Contact: <sip:[email protected]:10573>;+sip.instance="<urn:uuid:867E57BD-062C-7169-995D-C03CC0541C19>"
            Contact URI: sip:[email protected]:10573
                Contact URI User Part: tuer-iphone
                Contact URI Host Part: 192.0.0.1
                Contact URI Host Port: 10573
            Contact parameter: +sip.instance="<urn:uuid:867E57BD-062C-7169-995D-C03CC0541C19>"\r\n
        To: "tuer-iphone"<sip:[email protected]>
            SIP Display info: "tuer-iphone"
            SIP to address: sip:[email protected]
                SIP to address User Part: tuer-iphone
                SIP to address Host Part: foo.bar.net
        From: "tuer-iphone"<sip:[email protected]>;tag=ed68ae35
            SIP Display info: "tuer-iphone"
            SIP from address: sip:[email protected]
                SIP from address User Part: tuer-iphone
                SIP from address Host Part: foo.bar.net
            SIP from tag: ed68ae35
        Call-ID: 0RF7zFGABXSLst5gfmsDIw..
        [Generated Call-ID: 0RF7zFGABXSLst5gfmsDIw..]
        CSeq: 1 REGISTER
            Sequence Number: 1
            Method: REGISTER
        Expires: 60
        Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, REGISTER, SUBSCRIBE, INFO, PUBLISH
        Supported: replaces, answermode, eventlist, outbound, path
        User-Agent: VIDEOPHONE agent
        Allow-Events: hold, talk, conference
        Content-Length: 0

SIP-INVITE:

Internet Protocol Version 4, Src: 79.237.225.35, Dst: 192.0.0.1
    Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
    Total Length: 1255
    Identification: 0xf969 (63849)
    Flags: 0x0000
    Fragment offset: 0
    Time to live: 64
    Protocol: UDP (17)
    Header checksum: 0x8b8a [validation disabled]
    Source: 79.237.225.35
    Destination: 192.0.0.1
User Datagram Protocol, Src Port: 5060, Dst Port: 10168
    Source Port: 5060
    Destination Port: 10168
    Length: 1235
    Checksum: 0x2271 [unverified]
Session Initiation Protocol (INVITE)
    Request-Line: INVITE sip:[email protected]:10168 SIP/2.0
        Method: INVITE
        Request-URI: sip:[email protected]:10168
            Request-URI User Part: tuer-iphone
            Request-URI Host Part: 192.0.0.1
            Request-URI Host Port: 10168
        [Resent Packet: False]
    Message Header
        Via: SIP/2.0/UDP 79.237.225.35:5060;branch=z9hG4bKE574C2A06F46B037
            Transport: UDP
            Sent-by Address: 79.237.225.35
            Sent-by port: 5060
            Branch: z9hG4bKE574C2A06F46B037
        From: "Bauernstube" <sip:**[email protected]>;tag=BA5C24F75387FA12
            SIP Display info: "Bauernstube"
            SIP from address: sip:**[email protected]
                SIP from address User Part: **611
                SIP from address Host Part: fritz.box
            SIP from tag: BA5C24F75387FA12
        To: <sip:[email protected]:10168>;+sip.instance="<urn:uuid:867E57BD-062C-7169-995D-C03CC0541C19>"
            SIP to address: sip:[email protected]:10168
                SIP to address User Part: tuer-iphone
                SIP to address Host Part: 192.0.0.1
                SIP to address Host Port: 10168
        Call-ID: [email protected]
        [Generated Call-ID: [email protected]]
        CSeq: 25 INVITE
            Sequence Number: 25
            Method: INVITE
        Contact: <sip:[email protected]>
            Contact URI: sip:[email protected]
                Contact URI User Part: FB295F506401C13E567BBD9FC96B1
                Contact URI Host Part: 79.237.225.35
        Max-Forwards: 70
        P-Called-Party-ID: <sip:**[email protected]>
        Expires: 120
        Session-Expires: 600;refresher=uac
        Min-SE: 90
        User-Agent: AVM FRITZ!Box 7490 113.07.27 (May  4 2021)
        Supported: 100rel,replaces,timer
        Allow-Events: telephone-event,refer
        Allow: INVITE,ACK,OPTIONS,CANCEL,BYE,UPDATE,PRACK,INFO,SUBSCRIBE,NOTIFY,REFER,MESSAGE,PUBLISH
        Content-Type: application/sdp
        Accept: application/sdp, multipart/mixed
        Accept-Encoding: identity
        Content-Length:   359
    Message Body
        Session Description Protocol
            Session Description Protocol Version (v): 0
            Owner/Creator, Session Id (o): user 8940975 8940975 IN IP4 79.237.225.35
                Owner Username: user
                Session ID: 8940975
                Session Version: 8940975
                Owner Network Type: IN
                Owner Address Type: IP4
                Owner Address: 79.237.225.35
            Session Name (s): call
            Connection Information (c): IN IP4 79.237.225.35
                Connection Network Type: IN
                Connection Address Type: IP4
                Connection Address: 79.237.225.35
            Time Description, active time (t): 0 0
                Session Start Time: 0
                Session Stop Time: 0
            Media Description, name and address (m): audio 7080 RTP/AVP 8 0 2 102 100 99 97 101
                Media Type: audio
                Media Port: 7080
                Media Protocol: RTP/AVP
                Media Format: ITU-T G.711 PCMA
                Media Format: ITU-T G.711 PCMU
                Media Format: ITU-T G.721
                Media Format: DynamicRTP-Type-102
                Media Format: DynamicRTP-Type-100
                Media Format: DynamicRTP-Type-99
                Media Format: DynamicRTP-Type-97
                Media Format: DynamicRTP-Type-101
            Media Attribute (a): sendrecv
            Media Attribute (a): rtpmap:2 G726-32/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 2
                MIME Type: G726-32
                Sample Rate: 8000
            Media Attribute (a): rtpmap:102 G726-32/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 102
                MIME Type: G726-32
                Sample Rate: 8000
            Media Attribute (a): rtpmap:100 G726-40/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 100
                MIME Type: G726-40
                Sample Rate: 8000
            Media Attribute (a): rtpmap:99 G726-24/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 99
                MIME Type: G726-24
                Sample Rate: 8000
            Media Attribute (a): rtpmap:97 iLBC/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 97
                MIME Type: iLBC
                Sample Rate: 8000
            Media Attribute (a): fmtp:97 mode=30
                Media Attribute Fieldname: fmtp
                Media Format: 97 [iLBC]
                Media format specific parameters: mode=30
            Media Attribute (a): rtpmap:101 telephone-event/8000
                Media Attribute Fieldname: rtpmap
                Media Format: 101
                MIME Type: telephone-event
                Sample Rate: 8000
            Media Attribute (a): fmtp:101 0-15
                Media Attribute Fieldname: fmtp
                Media Format: 101 [telephone-event]
                Media format specific parameters: 0-15
            Media Attribute (a): rtcp:7081
                Media Attribute Fieldname: rtcp
                Media Attribute Value: 7081
            [Generated Call-ID: [email protected]]

Please note that this question is not about getting the app to work properly or configuring my router correctly. I'm mainly concerned with first determining which side is behaving incorrectly here so that I can then open a ticket with this company.

Solution 1:

Maybe both.

Search for settings like STUN or public IP (though I guess public IP would not work consistently as this would be dynamic IP) in softphone settings. If there is no STUN, refer to manufacturer.

Search through settings related to NAT in fritzbox. Common practice when dealing with clients from other networks is ignoring whatever is sent as Contact and using message source address instead. Refer to manufacturer if there are no settings for this.