English Gammer - Contact on/at

The use of "contact on" or contact at"


1) Pls feel free to contact our cumstomer service department "at" or "on" 3567xxxx.


My query is that some books use "contact on" tel no. and some books use "contact at" telephone" After all, which one is correct. I remember that my past business english tutor said we should use contact on rather than contact at but now I feel mixed up.