Hi Tom
Pretty confident the 'MaxRecordTime' value, from V4.1 found under
HKEY_LOCAL_MACHINE\SOFTWARE\AVAYA\Media Services\Directories
is the key that determines the max length of a recording.
The IP-Office \ Voicemail will actually carry out the recording to the limit defined by the registry key. If the recording is targeted to the VRL it is copied to a directory from which Contact Store will pickup the recording as delivered, and will not extend.
If a value of greater than 3600, say 7200 (= 2hours), is entered via the registry key, then the max record time of the recording delivered by the IP-Office \ Voicemail is extended. If the Voicemail Pro Client is started up, it limits the value shown to 3600, if this value gets written back to the server then the max record time on the server will again be limited to an hour.