[Asterisk-Users] simple question on asterisk

[Mark Hayward]

Hi,
I am planning to deploy an asterisk installation but I need to convince 
a few managers that its a good idea.
Theres something I don't quite understand though,
I plan deploy a box on the end of 4 channel BRI ISDN and provide it an 
ADSL internet connection.
Should a phone behind the asterisk PBX wish to call a VOIP phone number 
number, say an 0844 one from www.voip-user.org, would it send this 
automatically over the PSTN ISDN network or would it know to send the 
call over the internet.
Would I need a SIP provider on the internet to forward the calls? I 
assume I would need some sort of directory service to know where to 
route the call.
Thanks in advance,
Mark