asterisk / SIP calls dropping

Hi I have an issue where my calls in Asterisk will just drop randomly after a random amount of time,  asterisk seems to have a hissy fit and then the call just crashes out.  I have turn on advanced logging (Verbosity 5 Debug 5 SIP Debug on) and have attached a cut down version of the logs,  wondering if anyone could make any sense of them (Logs are just the tail of when twice this issue happened)
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Back up please --  Asterisk what, 1.4 or 11,  on what OS and through a Firewall ?  

We need a few details before looking at log files --
ncomperAuthor Commented:
Hi sorry this is hosted on the Amazon AWS cloud using their Firewall using NAT.  I.e Default external IP address and an internal IP address. The Version is "Asterisk (Ver.".  Im a little lost seems that people find it tough to read these logs :(.   Its running on the default Freepbx instance of Ubuntu which I believe is 10.0.4
I would start by talking with your provider and see what this series of messages is all about --  Way to many of them is the short log

<--- SIP read from UDP: --->
SIP/2.0 401 Unauthorized
Via: SIP/2.0/UDP;received=;branch=z9hG4bK76a29781;rport=5060
From: <>;tag=as3604d0b8
To: <>
Call-ID: 4785356a0d89474560bc2d29285711ff@
WWW-Authenticate: Digest realm="", nonce="c8a5a79899c0695e85d5e1120f9bc995", algorithm=MD5, opaque="1759901906"
Content-Length: 0

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ncomperAuthor Commented:
This did not 100% solve it but lead me down the right path
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