Hosted service is really no different than a typical hardware based system, and operates the same way. The key difference is the equipment is stored, and managed in a secure reliable location, with power backups in case of power failure.
No special equipment is needed, however; we do require your company's wiring infrastructure be CAT 5 or above, not sure just give us a call and we will schedule and evaluation.
Existing phone lines can be retained for devices like fax machines or security systems.
Existing phone numbers can be retained and reused with hosted solutions.
Typical installation can be accomplished within a week, and can be run in parallel with current installed systems providing very little downtime during the cut over.
Internet connection speeds/bandwidth needed would be based on the number of subscribers at any given location.
Most enterprise class networking equipment has the ability to provide VOIP service and can be reused.