Some companies might require an internet circuit. Maybe a cable modem. Some might require a larger circuit. It depends on how many employees you have and what your requirements are.
Typically, companies that are just starting, work on whats called a hosted PBX. We just install the phone system from one of the carriers. The good thing about that is that it can scale with you as you grow. If you have a goals in place for one year, two year, three year, you want to work with a carrier that can scale with you. That can be a hosted PBX, a cable modem, a larger internet circuit or SIP trunking.