When a user is actively handling a call in the RingCentral for Microsoft Teams integration, the arrival of a second incoming call overlays the current call control window and interrupts the user's workflow.
The issue is not that the second call is being offered. The issue is that the incoming call screen takes focus and replaces the active call control interface.
This creates a poor user experience in scenarios such as:

Transferring a caller to another employee
Searching for a transfer destination
Performing a warm transfer
Accessing call controls during an active conversation
Reviewing call information while speaking with a customer

If the user is midway through a transfer and a new call arrives, the overlay resets the call control screen. Even after clicking Ignore, the user must often restart the transfer process and re-search for the intended recipient.
Requested Enhancement
Allow incoming calls to be presented in a non-intrusive manner that does not replace or reset the active call control interface.
Examples could include:

A small notification banner/toast
A docked incoming call alert
An option to maintain the current call control screen in the foreground
A configurable setting for administrators or users to choose the preferred behavior