Your company has, since many years, a legacy PBX (e.g. Cisco, Avaya, Unify, …) and has recently adopted Microsoft Teams (as part of Microsoft 365 – cloud service) as internal collaboration tool. When calling internally, employees use their Teams client. When calling a customer or supplier, the employee uses this traditional (hard or soft) telephone set that comes with the PBX. Incoming calls arrive on this same telephone set. In some cases, these calls our intelligently routed by a call/contact center solution that is closely integrated with the PBX.
This situation is represented in the figure below: