I gave you an old number to contact me and I want to change the number because I have an appointment with you tomorrow
 

It might be better to contact 0800 55800 on the subject (to inform the technician involved about it)

Proximus moderators probably will not detect your message on this public forum on time  (due to the message handling delay time which probably will be 1 to 2 working days)

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Eventually also put your correct contact number in your forum login profile (free field ticket), there only visible for Proximus