Imagine a very heavy call volume situation where agent A answers a call and the customer asks specifically to speak with agent B. If transferred the customer may endure another long frustrating hold time. Politely advising the customer about the situation, agent A would suggest to transfer him to the voicemail of agent B instead. Agent B will return the call as soon as he is available, after hearing the message left by the customer.
Transfer to Voicemail is available as a dial code (the link refers to a Polycom model, but works on all brands.)