"at" is the right preposition to use in your sentence. You call a person "at" a certain telephone number. Another possibilty: "(Please) call me toll-free at ***."
You could just say "Please give me a call at your earliest your convenience."
Or something like (in the UK 'phone' is the more common verb than 'call') "Please phone at your convenience."
I like to finish off with something like "I look forward to hearing back from you." because it takes away any pressure they may have felt from you trying to get them to call you back (and usuall