(Composition of micro enterprise (our company)) I tried DIY during that time because it continues to work in-house (^_^;)
1. Subscribe to Office 365 to use Phone System
Business Premium → Enterprise E3 (Phone System added to this)
* If you are using Office365 Business, upgrade to Enterprise.However, since E5 is high, it was set to E3.
* Phone System is licensed to add only telephone functionshttps://www.microsoft.com/ja-jp/microsoft-365/business/compare-more-office-365-for-business-plans
2. Configure Session Border Controller (SBC) that links Teams application and existing PBX
Ribbon Communication SBC1000 is used this time as a Voice Gateway certified by MS (this is a little expensive)
3. Set on the Teams management screen
・Direct Routing setting in Teams
Associate an extension number with the registered device (BYOD mobile phone this time) and define it.https://docs.microsoft.com/ja-jp/microsoftteams/direct-routing-configure
(^ ▽ ^) What was done-Can extend the extension from the office fixed line to home
・Call/chat can be used separately on the same tool
・Since the smartphone/PC with the Teams app is an extension phone, the number of fixed phones is reduced.
・Search for contacts in the Teams app even if you do not know the extension number of the fixed phone/phone number of the smartphone
*If you want to do it, we will help you. (And light sales)