Firstly, hello everyone! 
Im quite new to telephone systems in general but i’ve been tasked with building an asterisk system for our new office (asterisk looks set to save us several thousand pounds 
), i’ve been reading up and asterisk itself looks fairly managable if a little migrane inducing to begin with, the one thing im uncertain on is the actual connection to the real world, hence i need a few things confirmed for me if, so if you fine people would be so kind as to let me know if im on the right path or just going nut’s id be eternaly greatfull!
Firstly, my planned setup, the phone line will be ISDN30 (BT’s pri service I beleive) with initialy 8 channels configured, im look at using the Wildcard TE110P to connect this to something like a dell sc1425 to provide phone functionality for 6 people, a fax line (via an smtp fax gateway) and one additional telephone line for general enquires, the phones will be grandstream budgettone100’s and the OS will most likley be gentoo.
The things im curious about are mostly do to with incomming calls, I beleive i’d need each channel configured with a DDI number, now i’ve read up and i know that the number asterisk listens on is the section of the number that varies for each phone line which i beleive is configured as the extension (can this be aliased to provide internal extension numbers ?), another thing im unsure on is outgoing calls, am i right in assuming each configured channel will be used by asterisk for outgoing calls on the number bound to it or do i need to do anything else to configure outgoing calls over the isdn ?
Ta in advance for any help at all!