Wonder if anyone can help. I have a situation where a customer works their way through an autoattendant on the embedded VM of a 3300. The last step transfers them to a ring group with 2 members. The call rings for 10 seconds and then overflows to a speed dial which sends the call to an offsite answering service. This works 99% of the time, but occasionally the caller ends back in the mailbox that made the transfer.
The mailbox COS is set so transfer recall is as high as it can go. The mailbox is set as mailbox because if its set as tranfer only, then it supervises the transfer and waits for an answer, whereas mailbox is supposed to do a blind transfer (as I understand it)
What else could be happening?

Have you timed the calls that fail? from what you describe it sounds like a recall- if the call bounces around you may exceed the timer.

Probably the Recall timer on the trunks. Check the COS for the truks and adjust the recall timer there.

Thanks guys,
sorry for the slow response but I was expecting an email when someone added to the thread, but that didn't happen.
I have adjusted the trunk COS and will see what happens.

Indeed the trunk recall timer is the one controlling the call.

The supervised/unsupervised transfer is a system wide feature. Use the admin mailbox to set the system wide transfer type.

The VM was on unsupervised, but when the recall timer kicks in the call will still recall to the voicemail.
Supervised transfer is far quicker to head back to the mailbox still, as the call is not actually transfered until the extension answers.