There are 3 places you put your leading digit that gets you an external line.
1/ System Options on the Controller
2/ Users and Services EHDU - External Hotdesking Dialling Prefix
3/ MiCollab Client Service - PBX Node Details - Dialling Prefix
For what you describe, only 1/ should be required......so not really sure what's going on there. Are you sure the number with the prefix is actually diallable from that phone?