This number can be configure in ini file at this location C:\Program Files\3CX&PhoneSystem\Bin\3CXPhoneSystem.ini incase you dont' like 9100
You would have to add
[DialCodes]
pickup = 9100
or
[DialCodes]
pickup = XXX
Have in mind that [DialCodes] category doesn't exist at the moment. You must create it and add pickup = xxx under it.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.