Thanks to all who responded. I am very grateful to you.
Particularly to DrGeoff, who wrote:
Physical Interfaces / Phone Port / Primary Line. Change its value to "Obitalk Service". Then all numbers dialled without a leading '*' or '#' will be sent to Obitalk.
This method sounds absolutely ideal, as the people using this system would not necessarily understand the need to dial prefixes. They just want something that looks, feels and works like a regular telephone system (except that it isn't a regular telephone system!).
Users will not need to make 'outside line' calls into the public network, as this project needs to act as a standalone, internal user only, system.
Again many thanks and Compliments of the Season.
Andy.