Before a call is transferred, it's important to tell the caller they are being transferred and tell the caller why they are being transferred. This ensures that the caller understands the reason for the transfer and can prepare for the next part of the call.
You may also consider placing the caller on hold for a short period while you connect them to the next party, but this is not universally required. Disconnecting the operator from the line is generally not necessary until the transfer is complete.
In summary, the best practice is to inform the caller of the transfer and the reason for it before proceeding.