On the market, you will find not only a wide variety of providers for telephone systems, but also many different systems with great differences in functionality, quality, look & feel, costs and much more. If you are buying a new telephone system now, you should think carefully about the range of functions it should offer you.
The price certainly plays an important role, but it is much more important that you can map your business processes on the new system. Try to test as many systems as possible in your evaluation process before making your final purchase decision.
Once you have decided on a system, look around for the right installation partner for you. This also needs to be chosen wisely, because this is where the wheat is separated from the chaff. The relationship between you as a customer and your installer should be based on trust. Remember that you will be working with this partner for the next 5 to 10 years.