We migrated a 150-user law firm from Avaya to Zoom Phone over a weekend. Here’s the unfiltered version of how it went.
The Plan
The cutover involved:
- Porting all DIDs from Avaya to Zoom
- Deploying Yealink T54W phones provisioned via Zoom
- Cutting over the FortiGate port6 to a new VLAN (
2026zoomphone, 10.20.30.0/23) - Training staff on Monday morning
We had a 72-hour change window starting Friday night.
What Went Wrong
SIP ALG (obviously)
Covered this in a separate log. FortiGate’s SIP ALG was rewriting headers. Disabled it early Saturday and that resolved call drops.
IP Blacklisting
Zoom blocked our public IP after a burst of failed registration attempts during initial provisioning. Had to contact Zoom support to whitelist us. Lost about 2 hours here.
UDP Port Range
The firewall policy wasn’t wide enough for Zoom’s media ports. Zoom Phone requires UDP 8001–65535 for media. We had a narrower range from the old Avaya config.
What I’d Do Differently
- Disable SIP ALG before the first registration attempt
- Pre-stage the firewall policy with the correct UDP range
- Do a single test phone registration 24h before cutover to catch IP blacklisting early
Result
By Sunday evening everything was stable. Monday morning was clean. Staff adapted faster than expected — Zoom’s interface is more intuitive than Avaya for most users.
Total downtime: ~4 hours, mostly on Saturday during troubleshooting.