Gramma:on/at telephone 9XX..?

You should just delete the word telephone. Everybody knows the numbers are your telephone.
If you have any queries, please contact me AT 9XXX XXXX
After some googling, I find that I'm using American english, but in uk, it's "on" everywhere.

That means:
USA:
If you have any queries, please contact me AT 9XXX XXXX
If you have any queries, please contact me on the mobile
UK:
If you have any queries, please contact me ON 9XXX XXXX
If you have any queries, please contact me on the mobile

If you have any queries, please contact me at telephone 9XXX


Plese call me at tel xxxxx when you arrive in Hong kong

We will call on (visit) you when we go to Shanghai next month

If you have any queries, please contact me at telephone 9XXX XXXX.

以上句子應用 at

2011-09-01 09:56:26 補充：
In the USA, you say "cell" instead of "mobile".

2011-09-01 10:04:29 補充：
Looking up the word "contact" in Cambridge dictionary, you'll find an example:
You can contact me UK on/US at (= speak to me by telephoning) 388 9146.
So, "on" is British way and "at" is American way of saying the sentence.

應該係用ON,因為你要在電話上面打號碼,如果後面冇號碼就唔知啦