This is a common fault when calls are directed to Virtual Extensions, not an issue for calls set to Ring groups ( as ring groups are never busy , even of there are no members.
A Virtual Extension VE by default an NEC database does not have call waiting enabled ( PRG 20-09-07 )
While a Virtual Extension is in “Ring” state, it is busy to all new calls, once that call has been answered a handset on any Line / CAP key the VE is no longer busy.
Enable call waiting for all classes