VoIP is not reliable, not for business.
One of our biggest customers is a major college near London and they have just spoken to us about installing new analogue lines everywhere as VoIP just isn't working. Just think if your broadband goes down you're stuffed and to be fair to BT, their lines don't go down that often.
The best thing is get a decent phone line with a decent provider (not BT) who wont charge you the earth for calls

. Can work out cheaper than VoIP and more reliable.