As usual Clough is correct about the mass of information concerning this.
I would just say that I would start by introducing your company, yourself and what exactly you do. (Keep this portion very short.) Then I would make three points about how you and your company can be the solution to any problems they may be having. I would pick three situations that are prominent problems in your industry and how you can service them.
First rule: KEEP it short, very short, people do not read much past the first paragraph. Fight the temptation to elaborate in this intro letter.
Ask for a response in the last paragraph (make sure that when they call back, they get a LIVE person) and state that your business isn't telecommunication...it is service and problem solving.
Another bit of advice; always say "thank you" in all situations even when they may complain....remember you always want to improve and even from complaints, you can learn something.
Please let me know if I may assist further,
Stringrer