I got this tip from Less is More.
Create an extension which goes directly to voicemail. When a solicitor calls up, transfer them to that extension and have a voicemail similar to:
Thank you for calling <Your Company Name>
Please listen to this message. Due to the high volume of calls received from outside vendors calling to solicitor thier products and services, we must ask that you forward in writing a description of your product or service along with any other information on your organization to:
This address can also be found on our website. <Website>
You can be assured that your information will be forwarded to the appropriate staff for review. If they find your company is of interest to our business, a staff member will contact you directly at a later date. Thank you for calling <Your Company Name> and thank you for understanding.