I try to identify who I'm calling (by company, job function, etc.) and what they tend to care about. Am I calling into Sales? I might refer to the bottom line, increased business generation capabilities. Procurement? Cost savings, thorough agreements, vendor relationships. HR? Internal difficult conversations.
Focus on who you're calling and their needs (some call them "pain points"). By speaking their language, briefly indicating that you can have an impact with them and that it's in their interest to call you back, you'll more likely receive returned phone calls.
If you're not getting call backs, consider new approaches (language, tone, etc.) and stick with it!
Stephen