Migrated 500+ Users Phone System off Avaya to RingCentral.

I had multiple clients with multiple PBX's. I migrated over 300 different companies from physical analog to VoIP setups. Yealink phones, cisco phones, doesn't matter. I've successfully done migrations of up to 500 users in 1 migration. The way I did it is to collect all extensions, DID's, Billing. Create the new setup, setup matching extensions and users. Create sip trunks to new providers for temporary config, make sure voice works as expected on premise. Then when ready port numbers to cloud providers.