I install phone systems and we use C-Talk for all our customers. C-Talk uses the Callplus platform and Phil is very very good to deal with. If you want cheap (ish) prices and great support Phil will sort you out. He is on here somewhere..
However, in terms of running 6-7 phones and wanting PABX type features then a hosted PBX is probably what you want. Or you could look at renting a phone system. Or you could get an old server and run Asterisk in house -probably the cheapest option in the long run as long as it is set up and maintained correctly.