Thanks Ralph, i'll definitely have a look into that, that sounds more preferable to what I was thinking..
The only other way I thought I might be able to achieve this would be to create a separate SIP peer profile for each customer name, all using the same outbount trunk. On the SIP peer profile you can control the default outbound CPN which could be the number of the company your representing. Then creating an ARS route using the specific peer profile for that customer and leading digit plan for each. This way seems like it could get a bit messy though with all the digits dialled and ARS route entries required.
Martin