I have had a stable system for well over 1 1/2 months now, and now out of the blue - completly unusable - I am trying to hunt down the problem - first it started like this:
Transmit was garbled - Rx was fine, but could not understand voicemails or anything leaving the asterisk system.
Phones can call the asterisk server, but no other SIP phones.
Noticed a major network problem (bad nic) in another server causing massive network ISSUES! - unplugged it - restarted all devices on network - still phones can not call phones and do not see the asterisk server.
Incomming call come in, but do not find their way to the extensions, unless they are handled by the automated attendant.
I have tried to locate any network loops or issues, but I cant see this being a huge issue because all the phones are on their own VLAN… so I am now going phone by phone to see if I can see anything wierd…
Any suggestions??!!
Update - I can not ping any of my DHCP clients from my asterisk box?! I have no idea why! - I can ping anything else on my network… I can ping them from other Linux/win boxes on the same switch!
What is going on? – I have like 150 very angry people right now 
Hrmm this is not good…
Update - things just getting worse - I can not ping ANY phone from the Asterisk server. They are not provisioning any more - they get a DHCP IP and then just hang.
I can ping the phones from other linux servers – I am at a LOSS!
Still broke??
stop asterisk and start it with asterisk -cvvvv
does it start clean or hang up?
Version of asterisk??
if hanging at app_trunkisavail.so known bug
fix is to remove the app_trunkisavail.so and add this “chan_zap.so => yes” to the [global] section of modules.conf
Have a look at the processor usage.
Check which processes are running, look for multiple processes.
Check disk space usage and the logs.
OK got it fixed here was the problem:
we use a PRI here, but all phones obviously go over our LAN.
The NIC in our gateway router was freaking out causing bad things to happen (constant resend of packets, ERRORS, ERRORS, ERRORS), and when the Polycom phones attemped to provision as soon as they recieved the Gatway IP address from our DHCP server, the phones attemped to contact the Gateway. Because of the massive amount of errors when trying to contact the gateway, the phones would stop the provisioning process. and hang thus not starting up and not registering with the Asterisk server. Then entire time I had no errors on our asterisk box, or from the phone logs
VERY ANNOYING!
Now our bad NIC is out of the network all is well, jsut reboot all the phones and everything is great 
– I just wish the polycom phones would say something in their provisinoing log - instead I had to do packet traces/tcpdumps to figure out whats going on!
So note to self and everyone else - Polycom phones need to have a “working” gateway IP in order to provision (not to say then need to have access to the internet, but if you have an IP in the gateway address it needs to be functional).
This was a freaking pain to figure out! I need to get some good network monitoring tools - anyone have any suggestions - I have all HP equip so I was thinking HPs network managment tools.