'At' or 'on' before a phone number?

Both are correct. In British English, it is more common to use 'on', while 'at' is more commonly used in American English.



 Contact me on 2288 2288 for your appointment.

 You can reach us at 2288 2288 for your appointment.


However, no preposition is needed for 'call (a number)':

 Call 2288 2288 for enquiries.


Also, we usually use 'at' before email addresses.

 Please email Pauline at p1@abc.hk for enquiries.