This might be a stupid question from my side and my intend is not to waste your time but just trying to think along since most other options have been discussed by others.
I don't know exactly which parameters you have checked, but do you have a feature access code programmed for:
- Conference call
- Conference call split
Under System Properties -> System Feature Settings -> Feature Access Codes
Because it seems in what you describe conferencing as a function doesn't respond at all which led me to this one.
If for some unknown reason they are empty fill them with non-conflicting entries e.g. *40 and *41 and try again.