title: Twilio elastic SIP trunks with Avaya system
body: <p>Hi if anyone else ever needs help connecting an Avaya IP Office phone system with Twilio Elastic SIP trunking, feel free to reach out to me, we got it to work after some trial and error and the connection seems very good.  Will save us some good money.</p>
  
  <p>The final piece to the puzzle for us had to do with the IP address that Twilio sends SIP invites from for incoming calls to the Avaya system.  To get this to work in the Avaya we had to set up 4 SIP "Lines" pointing to 4 different "ITSP IP Address" values and 4 "Incoming Call Routes" per DID because Twilio sends from 4 different domestic IP addresses.  I did read some documentation about being able to leave the ITSP IP Address blank (0.0.0.0) since we do have the "ITSP Domain Name" populated but that does not seem to work in our situation.</p>
  
  <p>In researching this on the Internet I surprisingly did not find much evidence of Avaya customers ever having to deal with this issue (with or without Twilio), however I did find some examples of some other PBX systems having this issue with connecting to Twilio but those systems seemed equipped to configure multiple IP addresses for a single line.</p>
  
  <p>Thus my question - do any Avaya customers have a suggestion for an alternate configuration to make this work?  Anything we could do on our firewall?  Twilio does not support sending SIP Invites from a single IP address or via a proxy server address.  Fortunately we only have about 10 DID's so this won't be that hard to setup and maintain.</p>