One approach can be to use a shared directory of external contacts. Please see link below.
Creating a shared directory of external contacts – Zoom Support
Another approach is to get the new desk phones configured and installed piecemeal and then port all the numbers at one time. Not sure if it will work for you because your numbers may be much more than the 400 users in my example.
A very small IT department with two members assigned part time and assistance from Zoom Professional Services converted 400 users and 31 bank branches from Avaya IP Office.
Existing Avaya digital phones were not compatible with Zoom Phone so bank installed Poly VVX phones at beginning of conversion. These Poly phones used the same extension numbers as were assigned on the Avaya system and same DID numbers as were assigned on the Avaya system. In addition to existing DID numbers, temporary DID numbers were assigned for testing.
Users were encouraged to use the Poly phones for internal calls and for testing incoming and outgoing calls.
All the DID numbers were ported from Avaya system to Zoom 90 days after signing the Zoom contracts.
The Avaya phones stopped ringing and the Poly phones began ringing. When the porting took place, the temporary numbers were automatically replaced with the ported numbers.
We expected someone to report some issue, but there were no reported issues.
