The advantages are around cost and flexibility.  VOIP based phone systems are not tied by local office wiring like in a conventional phone system.  Calls can come into your Florida based office, and ring at the home office of your California based employee.  That can can also be routed via the public network out to an office in Germany for no additional cost.

SIP based VOIP service tends to be most flexible.  You can obtain SIP trunks (inbound/outbound service to public phones) can originate in any country or location.  You can have local phone calls world wide with this service if you find most of your customers/vendors in a different area of the world.

Many businesses are using SIP TRUNKs to create their own inexpensive digital PBX units.  With SIP trunks, your business can have a "local" phone number in any country or state.  If you are making frequent calls to certain locations, this may be the best solution for your business.

The draw back for VOIP based service is the Quality of Service.  Given the use of the public network, you could experience quality problems.  These issues will not bother most customers, worse case you might need to place the call again if the service isn't clear.  The Quality is also based upon the company/user connection to the network.  Low latency services with high bandwidth would be preferred to cope with additional network traffic such as uploads/downloads.