Do We Need A Phone System ?

In your shoes I would definitely not invest in a phone system. I would sign up with a hosted or virtual phone service. Lines can be added or disconnected as business conditions change. You pay for what you use. There is no capex or locks in's to technology. Virtual phone services commonly provide features and services that premise based solutions don't. Even better, let someone else take responsibility for maintaining the service. Once the operation has been going for a year or 2 you can review again, but typically you can expect to save heaps by using hosted services.