Everyone you call should be followed up with. They will fall in to one of four categories.
Most of the time, calls will result in a voicemail (or no answer if they do not have a voicemail set up or it's full.) Let me be the first to tell you something that you'll hear repeated countless times — THEY AREN'T IGNORING YOU. Remember, calltime opens us up to our deepest psychological fears. Voicemails can feel like rejection, but they aren't. Instead, they're the evidence that people are busy, they have a lot to do, and that your priority #1 is their priority #328. That's why we have a system to stay top of mind.
Occasionally you'll get someone who wants to call you back at a later date. Often they'll say things like "hey can I call you back after lunch" or "let me talk to my wife and call you back". Your job in these circumstances is to maintain control. Instead of saying "sure, I look forward to hearing from you", say "sure I'll call you back at 1:30" or "sure, how about Thursday at 5:00". You should always be in control of time.
See Pledge Chasing for the detailed process of how to bring in the money.
Following up with donors is what "Follow ups" is all about. After they donate, donors need to be followed up with and provided information about your campaign or organization that really adds value. Keep them abreast of what is going on, host events that bring your donors together, bring in special guest who your donors will appreciate. Following up can take many forms, but at the end of the day it is about adding value for your donors.