@Arnold+1904
Any business phone system might well have features which are capable of generating that effect for inbound calls to the host end.
Disabling (or misconfiguring) Call Waiting might do so, as could some configurations of Call Forward When Busy.
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Sky Glass 55" (on ethernet) & two Stream Pucks (one ethernet / one WiFi)
BT Halo 3+ Ultrafast FTTP (500Mbs), BT Smart Hub 2
