The Skylight product is indicating that the agent is receiving a call, however, the desktop phone does not ring.
There may be a carrier hand-off problem between the Serenova carrier (Twilio) and the LEC (local exchange carrier) connected to your PBX.
In these cases you can reach out to Twilio and ask them to identify the carrier for the location and change to a different production voice carrier to resolve the hand-off problems experienced.