We just started using skype as we are out of the country.
$18 for a phone
number for 3 months and something like $0.02/min for calls and it comes with voice mail.
The big issue with VOIP systems is you need an internet
connection that isn't data limited. You can get away with an occasional call using a cellular data card but if you are conducting buisness with lots of calls, it can get expense (though not as bad as $1-2min international calls).
Currently we are in an apartment with unlimited wifi
, so it works great.