What Is the Best VOIP Service for My Small Business (12 Persons) in New York?

If you have some networking technical expertise, you can save substantially by installing your own VoIP ATA (analog telephone adapters) or an IP-PBX.

I like using the Linksys SPA2102-NA 2-line ATA's configured to connect to VoIP carriers like CallCentric.
http://www.linksys.com/servlet/Satellite?c=L_Product_C2&childpagename=US%2FLayout&cid=1146582254856&pagename=Linksys%2FCommon%2FVisitorWrapper&lid=5485693890B02

However, the Linksys SPA9000 is an IP-PBX appliance that is licensed for 4 configurable lines and license expandable up to 16 lines.
http://www.linksys.com/servlet/Satellite?c=L_Product_C2&childpagename=US%2FLayout&cid=1139414817110&pagename=Linksys%2FCommon%2FVisitorWrapper

You may also want to look into TalkSwitch hybrid and IP-PBX appliances:
http://www.talkswitch.com/us/en/products/phoneSystems/244vs.aspx

Then, there is the option of installing your own IP-PBX server solution using IP-PBX software from 3CX corp:
http://www.3cx.com/

Any of the above solutions are user hardware and software in-house solutions that require some networking expertise on the user side.

Then, it is just a matter of finding a BYOD (bring your own device) VoIP provider like CallCentric, InPhonex, or Les.net. These are VoIP carriers that allow your to supply your own equipment, configure it yourself and connect to their carrier services through your VoIP account. These companies do not require contracts and are strictly pay-as-you-go pre-paid calling VoIP solutions.

If you don't have some technical expertise for setting up your own VoIP systems in-house, then you may want to consider a Hosted VoIP solution by Packet8.