In a supervised transfer setting, it is important to make sure that the PhoneServer call timeout setting (Edit->Configuration->Transfer Settings) is less than the PBX setting for forward on No Answer. If not, while MailCall is still waiting for the call to complete the PBX will transfer it back to voice mail. This usually means the second port in the voice mail system. The first port is happy because it thinks the call has been completed (assuming no call screening). The second port thinks this is a new call and the caller thinks they have been put back to the main menu after trying an extension.