Don't just think phones when looking for a new phone system.
Depending on the size of your business, the cost of a phone system works out about the same price as a high end PC for every 4 users, which works out about the price of a low end PC for every employee. The cost per head starts to drop significantly the more employees you have.
Not a bad investment considering that most of your customers or vendors contact your via the phone and, unlike PC's that are almost obsolete within 3-4 years, a new phone system will give you good service for 10 plus years.
With a phone system, unlike PC's, it has the ability to use new technologies with simple additions. Take VoIP, if there is a real cost benefit for you to use it, like saving on long distance charges, a simple bridge will connect yon to the service. You can even bridge you office phone system to your cell phone provider, enabling you to call the office or have calls transferred to you without using your call minutes. Call a customer through the cell bridge and the customer will see your office number, not your cell phone number.
The phone system will probably be the main entrance to your company. Adding third party products to your system can enhance your companies professional image and user efficiencies. Products like Music-on-Hold can promote your company, Call Analyst products can provide valuable information for marketing campaigns.