For a small business owner when considering the transition from traditional premise based PSTN phone systems to IP based Hosted PBX System what are the key points one should be aware of?

Business communication is a crucial factor that can greatly increase your enterprise image and get you more customers. A business class telecommunication system is a must for every organization. But small business enterprises can not afford an expensive business phone system.
Today, more and more SMBs are switching to Hosted PBX Systems as cost effective business communication systems that offer a number of features and have minimum IT requirements.
But before you sign up for any Service Provider, there are a few points that should be kept in mind.
1. What is the QoS (Quality of Service)?
2. What would be the upgrading costs involved if my organization grows past its current capabilities? Can your system meet my growing business needs?
3. What is the uptime provided? In case of a power failure or an unplanned disaster, what are the disaster recovery plans?
4. Do you support emergency 911 calling?
5. What is the termination policy?
6. Do you provide technical support? Is it free or do I have to pay extra for it?
These are some of the facts you should know before making the decision.

can you explained these tirm m a newbie don't get this
PSTN phone systems
IP based Hosted PBX System

PSTN phone systems
IP based Hosted PBX System is old system.