I would use "on" in all three sentences.
a. You can reach me on 0907777999.
b. You can call me on this number 0907777999.
c. You can contact me on this number 0907777999.
"At" is probably possible for all three too.
Here's how I'd use your prepositions:
You can reach me at home or at the office.
You can call me with a phone card.
You can contact me through my secretary.