[Q] Native SIP-Client with FRITZ!Box-Router - Android Q&A, Help & Troubleshooting

Hi,
I'm having some trouble getting the native Android SIP client to run with the builtin SIP server of my FRITZ!Box router. The native client works great with my Sipgate-Account, however it doesn't want to stay connected to my router. I've made a Wireshark trace which looks like this:
{
"lightbox_close": "Close",
"lightbox_next": "Next",
"lightbox_previous": "Previous",
"lightbox_error": "The requested content cannot be loaded. Please try again later.",
"lightbox_start_slideshow": "Start slideshow",
"lightbox_stop_slideshow": "Stop slideshow",
"lightbox_full_screen": "Full screen",
"lightbox_thumbnails": "Thumbnails",
"lightbox_download": "Download",
"lightbox_share": "Share",
"lightbox_zoom": "Zoom",
"lightbox_new_window": "New window",
"lightbox_toggle_sidebar": "Toggle sidebar"
}
Sipgate account: REGISTER -> 401 -> REGISTER with user/pw -> 200 -> OPTIONS -> 200
FRITZ!Box account: REGISTER -> 401 -> REGISTER with user/pw -> 200 -> OPTIONS -> 401
The FRITZ!Box account works with other soft phones like PhonerLite.

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startaq said:
I'm having some trouble getting the native Android SIP client to run with the builtin SIP server of my FRITZ!Box router. The native client works great with my Sipgate-Account, however it doesn't want to stay connected to my router.
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I know this is an old thread, but did you manage to get things to work in the meantime?
I've just started trying the SIP functionality of my FRITZ!Box router with CM 12.1, so native Android SIP stack. I can get registration to work and phones to ring, but the moment I pick up (incoming through FritzBox) or the callee picks up (outgoing through FritzBox), the connection is broken. Perhaps your experience can halp me diagnose my issue.