ADVICE Please! HOW do I get Advertising Agencies to take notice of my service?

I have a business that provides an OUT OF HOME advetising and promotional service for any type of business.
I’m looking for information, tips and advice on how to get advertising agencies intrested and take notice of our very effective and unique service and make them relize the impact potential for their clients.
I have sent out information packs and contacted several agencies but to no avail, they cannot even return phone calls.
I welcome any and all advice please
I would like agencies to consider using our service for their clients campaigns.