re: VOIP for business?

Small office with 4 users, iPhones for all. Will have 6 phones in facility in all, and like the feature of having the iPhones appear as "office" phones when calling/answering from job sites or on the road.

Currently have a pbX system with voice mail through ATT landline. Also have Charter Internet 6ombps.

So, to recap, we would like 5 desktop phones, 1 breakroom phone, 1 fax line (and yes, some of our clients still fax), and integrate 4 iPhones into the mix. I was looking for a reliable VoIP carrier and provider to use with our Charter high speed cable internet.

Thanks for the input.

John