I figured out that there is some sort of incompatibility with snom and
asterisk's sip.
For the first time the authentication looks like:
NOTICE[5126]: File chan_sip.c, Line 4424 (handle_request): Failed to
authenticate user <sip:800@157.181.25.113>;tag=yiubra2azl for SUBSCRIBE
NOTICE[5126]: File chan_sip.c, Line 4486 (handle_request): Registration from
'"Levi" <sip:800@157.181.25.113>' failed for
'62.112.220.107'
NOTICE[5126]: File chan_sip.c, Line 4424 (handle_request): Failed to
authenticate user <sip:800@157.181.25.113>;tag=yiubra2azl for SUBSCRIBE
NOTICE[5126]: File chan_sip.c, Line 4486 (handle_request): Registration from
'"Levi" <sip:800@157.181.25.113>' failed for
'62.112.220.107'
    -- Registered SIP '800' at 62.112.220.107 port 5060 expires 60
And after this expires I just keep getting the following messages 10-50 per
registration period:
NOTICE[5126]: File chan_sip.c, Line 4486 (handle_request): Registration from
'"Levi" <sip:800@157.181.25.113>' failed for
'62.112.220.107'
NOTICE[5126]: File chan_sip.c, Line 4486 (handle_request): Registration from
'"Levi" <sip:800@157.181.25.113>' failed for
'62.112.220.107'
NOTICE[5126]: File chan_sip.c, Line 4486 (handle_request): Registration from
'"Levi" <sip:800@157.181.25.113>' failed for
'62.112.220.107'
What can be the problem? 
Cos usually after a few minutes it registers itself again. And so on. But in the
time the phone is unregistered it can't reveice any calls.
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