Note what you can offer them or their organization. If you were referred by someone they know, say so. Project energy and enthusiasm. It can often be difficult and frustrating to reach people when you are networking. Sometimes 10-15 calls will be required, but be persistent.
Managers are busy people.
Don't take it personally if they don't respond. Their work demands may keep them returning your call, even though they would be glad to talk with you. Most individuals understand and even appreciate repeated calls. Such initiative indicates genuine interest and motivation on your part. If your calls become annoying, people will let you know.