The users will be homed on Office 365 with voice breakout remaining on-prem for the moment.
After moving one user saw the following strangeness:
Usually the number will normalizes nicely to E164 format but for Jo it hadn't. When drilling down into the user account had the following message:
"The country/region codes of the online dial plan and the telephone number don't match so international calling might not work as expected"
Interestingly Jo could make and receive PSTN calls with no issue (in region, didn't try international) so it just seemed cosmetic.
On looking at the account in the on-prem SfB control panel the only difference from other users who had been moved correctly was that the Tel had an upper case "T":
I changed this to a lower case "t", forced a sync to O365 and this did not solve the problem. I'm making a stab that the sync process did not see this as a change worthy of uploading.
To fix I changed the on-prem number in the SfB Control Panel to not have the ext attribute and forced a sync:
The changed back to the correct lineuri:
After one final sync all was showing up correctly: