asterisk users - Jun 2014 - s4 built in sip client and 481 call/transation does not exist error

Hello

i'm experimenting a bit with asterisk to see if i can get it to work 
they way i want it to.

i'm no asterisk expert and i've run into a bit of a problem that i
can't
figure out what is wrong.

what i'm trying to do is to use my mobile phones built in sip client (a 
samsung s4 phone) to connect to asterisk.
this is directly over wifi to the asterisk box so there is no nat or 
anything like that that might cause issues.

the problem i have is when i use the mobile phone to make a sip call to 
another extension and i hang up before the call is answered the phone 
never stops ringing.
when i use wireshark to see whats going on it appears that when the 
mobile phone sends a sip cancel message asterisk replies with 481 
call/transation does not exist for some reason.

any idea why that is? and what is going wrong?
i can probably provide a packet dump if needed.

the strange part is that if i on the mobile phone enable "receive 
incoming calls" and then remove it, everything works just fine.
it continues to work even after i have rebooted the mobile phone or 
rebooted the asterisk box.
so once it is in a working state it stays like that for about 30 minutes 
or so.
this also makes testing a bit hard since i cant force it to fail easily 
once i got it to work.

yes if i use another separate sip client on the android phone it always 
works but i wanted to see if i could get the built in client working 
since it probably have good integration with the rest of the phone and 
it is already there.

one thing i noticed during my testing was that once the phone is 
registered with asterisk, asterisk keeps sending periodic sip messages 
and any icmp destination unreachable packets it receives appears to be 
ignored.

my asterisk box is using the freepbx distro (asterisk 11.9.0)