It really depends on the number of devices and lines in the sites, we move each site in sequence. For the migration of dedicated customers there will be very limited downtime as the new dial plan is deployed in parallel. When the devices and lines are migrated the only down time is while the phones re-register, there will be slightly longer downtime for services such as hunt groups and pickup groups while all the members migrate. Feel free to contact us (contact-us AT kurmi-software.com) to continue this discussion.