Callers waiting in the queue are not offered an option to leave a voicemail.
Virtual Contact Center
VCC Configuration Manager portal: Check the Queues/Skills > Edit Inbound Phone > Voicemail tab to confirm that voicemail is enabled or in the IVR script that there is an overload to a VM queue.
This issue is generally caused by voicemail not being enabled in Queues/Skills > Edit Inbound Phone > Voicemail or the IVR script does not offer the caller an option to leave a voicemail (i.e. missing overload to voicemail or forwarding to a queue that has VM option disabled).