After a lot of searching I ended up picking a service called Toktumi (pronounced Talk to Me) and I have been really happy with the service provided.
They have a web interface where I can add/remove extensions, see my call log, listen to voicemails, etc. Another awesome feature is they have an app for my Blackberry and iPad that allows me to make calls, text messages and listen to voicemails. It really makes life easy and I can easily forward the number to someone else if I am out of town or busy in meetings all day.
My only complaint is I wish they had an easier way to dial outgoing numbers. Maybe they do and I don’t know about it. But basically I have to first dial my number, hit 2 and then dial whatever number I want for the Caller ID to come up as my rental number. It is kind of a pain to do from my car.
Check it out, Toktumi.