Explanation/Reference:
Explanation:
Phone System in Office 365 (Cloud PBX) is Microsoft's technology for enabling call control and PBX capabilities in the Office 365 cloud with Skype for Business Online. Phone System allows you to replace your existing PBX system with a set of features delivered from Office 365 and tightly integrated into the company's cloud productivity experience.
Note: To enable calls to landlines and mobile phones, Phone System can be connected to the PSTN in one of two ways:
By purchasing Calling Plan-the PSTN calling plan add-on to Office 365.

By using on-premises PSTN connectivity, where software on-premises connects with your existing

telephony infrastructure.
References: https://docs.microsoft.com/en-us/skypeforbusiness/skype-for-business-hybrid-solutions/plan- your-phone-system-cloud-pbx-solution/plan-your-phone-system-cloud-pbx-solution