I'm trying to connect my E65 to the sipgate.de VoIP service using the built-in client, however it seems to be unable to establish a connection. I started investigating the issue a bit myself and while I couldn't pin down the cause of the problem due to lack of access to a tcpdump-style application for S60 to confirm that UDP packages don't get lost anywhere after passing the WLAN-router, I do think that the problem is to be looked for on the device itself.
I captured packages on the router's WLAN device in order to take a look at the communication between phone and sipgate.de and the results show that a packages sent by the E65 do indeed reach the sipgate server, however the response is either not or incorrectly processed by the SIP implementation on the phone.
To initiate a connection, a REGISTER request is sent to the server but apparently the response, a "401 Unauthorized", is not processed but instead the REGISTER request is repeated in increasing intervals (3s, 6s, 12s,..). According the the RFC, the device should resend the request with credentials added and CSeq increased, however nothing inside the request gets changed for the next send.
The response packages are sent to the phone over the WLAN device on the router so I assume the problem doesn't lie here.

I'm a bit out of ideas of how to fix this issue without debugging access to the SIP application on the E65 but maybe there's something I'm missing and someone here has a solution for the problem.
Would be great since VoIP was one of the main reasons for me to buy this phone

Part of the capture:
Code:
No.     Time        Source                Destination           Protocol Info
      1 0.000000    84.191.201.140        217.10.79.9           SIP      Request: REGISTER sip:sipgate.de;transport=UDP

Frame 1 (470 bytes on wire, 470 bytes captured)
Linux cooked capture
Internet Protocol, Src: 84.191.201.140 (84.191.201.140), Dst: 217.10.79.9 (217.10.79.9)
User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 5060 (5060)
Session Initiation Protocol
    Request-Line: REGISTER sip:sipgate.de;transport=UDP SIP/2.0
    Message Header
        Via: SIP/2.0/UDP 84.191.201.140:5060;branch=z9hG4bKetou3ao40lhc6glt0g5ga31;rport
        From: <sip:[removed]@sipgate.de>;tag=4vce3auojphc7aka0g5g
        To: <sip:[removed]@sipgate.de>
        Contact: <sip:[removed]@84.191.201.140:5060;transport=UDP>;expires=3600
        CSeq: 867 REGISTER
        Call-ID: 9LXxGX1PoIcsLwgb1pDkI13Q3z94AA
        User-Agent: Nokia RM-208 1.0633.18.01
        Max-Forwards: 70
        Content-Length: 0

No.     Time        Source                Destination           Protocol Info
      2 0.072174    217.10.79.9           84.191.201.140        SIP      Status: 401 Unauthorized    (0 bindings)

Frame 2 (463 bytes on wire, 463 bytes captured)
Linux cooked capture
Internet Protocol, Src: 217.10.79.9 (217.10.79.9), Dst: 84.191.201.140 (84.191.201.140)
User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 5060 (5060)
Session Initiation Protocol
    Status-Line: SIP/2.0 401 Unauthorized
    Message Header
        Via: SIP/2.0/UDP 84.191.201.140:5060;branch=z9hG4bKetou3ao40lhc6glt0g5ga31;rport=5060
        From: <sip:[removed]@sipgate.de>;tag=4vce3auojphc7aka0g5g
        To: <sip:[removed]@sipgate.de>;tag=b11cb9bb270104b49a99a995b8c68544.f30e
        CSeq: 867 REGISTER
        Call-ID: 9LXxGX1PoIcsLwgb1pDkI13Q3z94AA
        WWW-Authenticate: Digest realm="sipgate.de", nonce="45e5e7ebf6d3cc7e078e8295ba71a9092ea68bc7"
        Content-Length: 0

No.     Time        Source                Destination           Protocol Info
      3 3.016895    84.191.201.140        217.10.79.9           SIP      Request: REGISTER sip:sipgate.de;transport=UDP

Frame 3 (470 bytes on wire, 470 bytes captured)
Linux cooked capture
Internet Protocol, Src: 84.191.201.140 (84.191.201.140), Dst: 217.10.79.9 (217.10.79.9)
User Datagram Protocol, Src Port: 5060 (5060), Dst Port: 5060 (5060)
Session Initiation Protocol
    Request-Line: REGISTER sip:sipgate.de;transport=UDP SIP/2.0
    Message Header
        Via: SIP/2.0/UDP 84.191.201.140:5060;branch=z9hG4bKetou3ao40lhc6glt0g5ga31;rport
        From: <sip:[removed]@sipgate.de>;tag=4vce3auojphc7aka0g5g
        To: <sip:[removed]@sipgate.de>
        Contact: <sip:[removed]@84.191.201.140:5060;transport=UDP>;expires=3600
        CSeq: 867 REGISTER
        Call-ID: 9LXxGX1PoIcsLwgb1pDkI13Q3z94AA
        User-Agent: Nokia RM-208 1.0633.18.01
        Max-Forwards: 70
        Content-Length: 0

[...]