If you are starting up your own business, you will need to think about a phone system. This would include telephones for making and receiving calls, and lines for voice and data (ie internet).
You may choose to have a traditional line for voice calls, and a separate line for data, or you may consider using VoIP (Voice over IP, basically voice calls converted to data and sent via the internet), for which you need only one type of line. However, this will mean you will have requirement for substantial upload capacity of data, so you might think about using symmetric digital subscriber line (SDSL), which has the same upload as download capacity, rather than the traditional asymmetric line (ADSL). Regardless of whether you choose VoIP, consider your upload data requirements as your needs may be far in excess of normal residential usage.
What suits you best within your office will depend on how many users you have. For larger installations, a PBX is probably most suitable, while for a few users, a key system is enough. For a very few users (up to five, perhaps), a KSU-less system (where capability is within each phone, rather than in a central unit) could suffice. A VoIP system works differently again.
There are many vendors that can provide you with systems. You may wish to just buy BT Synergy business phones, a well respected brand. However, given that there are a myriad of different solutions, a little research and good advice may avoid a poor and costly selection. Consultancies such as Brite Telecom can not only supply you with a phone system, they can present you with options for a solution and help you decide what you actually need.