Exam MS-720 topic 1 question 17 discussion

You have a Microsoft Teams deployment and an existing telephony solution that provides PSTN capability to on-premises third-party PBX systems in two separate locations.
You need to implement a redundant PSTN solution for Microsoft Teams Phone. The solution must use the existing telephony carrier services. The solution must ensure that outbound calls can be made if a component fails.
Which three components should you implement? Each correct answer presents part of the solution.
NOTE: Each correct selection is worth one point.