One thing which is sort of related that we learned the hard way - don't have sip accounts that match the phone number. An example of this would be if you have an Asterisk running as a sip server that provides DID service to customer owned Asterisk servers. Lets say you had a customer with a DID number coming into your main server over a Dahdi PRI circuit and the number was 2485551234. If you create a sip account for that customer called 2485551234 that will act as a sip trunk between your server and his server, no calls can be created on your server from anywhere (Dahdi, sip, iax, etc ...) to that number because it matches the name of the sip account. Instead you can create the sip account name like this: "cust51234" or something of that nature. Doing it this way not only negates the problem I described but also adds a bit of security because your mixing alpha characters into the name.