I believe that with attendant transfer there is a timeout in the system that pulls the call back before the voicemail hits (something like 10 secs i believe). You can increase the timeout for attendant transfers but i think the issue that you will run into is that with users that dont have call forwarding, instead of the call being pulled back to person transferring the calls automatically, you will get their voicemail. If you want to give it a shot then add the following line to the /etc/asterisk/features_general_custom.conf:
atxfernoanswertimeout = 60
you can play with the secs (60 in this example) and see if it helps.