Implementing VoIP in an Organization with 70 Users

I'm new to VoIP and would like to look into implementing a solution to cut down on our long distance call costs since we have such a high volume of outbound LD calls to our customers, remote associates and corporate office. My first question, using VoIP will LD calls be virtually free or is there a fee when switched to the PSTN? What are some things to consider? Any sites with information would be helpful.

Thanks.