I am just confused as to when SIP trunking is required. Lets say i have an organization with 100 nodes and we want to deploy centralized call manager at hub. Now what i am confused about is, do i ever need a sip trunk between my own gateways even when i am not communicating with any ITSP ? Like in my case i have 100 routers in all 100 nodes. Call mades on voip will be forwarded by these routers, so if we focus on the calls that are made within an organization, do i still need sip trunks anywhere ?
Pls clear it for me
That's a true statement.
You have vastly underestimated the role of the call agent in VoIP and I strongly suggest reading the SRND document I included previously. You're missing several large concepts.