Business plans, like Office 365 Business Premium and Microsoft 365 Business do not support the phone system and calling plans. Phone systems and calling plans add-ons are for Enterprise plan only.
If you have users that are on a "Business" plan, they will need to be upgraded to an Enterprise plan. Here are 2 options with security in mind...
Options no 1: We recommend upgrading to Microsoft 365 E5 ($57.50) which includes all the Office apps, security features, and Windows Enterprise. Then add a calling plan add-on
Option no 2: I know no 1 this is a steep cost increase. The alternative is to upgrade him to Office 365 E5 ($35) and add EMS E3 ($8.75) to preserve the security features that came with his former M365 Business plan. Then add a calling plan add-on.
I hope this help, let me know if I can be any further assistance.