<p>Are fax messages and voicemails stored encrypted?</p>

No, all messages are stored on the storage medium (USB stick or SD card) unencrypted. The storage media of COMpact 4000/5000/5200/5500 and COMmander 6000 have the ext4 format and are therefore usually not readable on a Windows PC without additional software.
The storage media of the system telephones are in FAT/FAT32 format and can be read out on a Windows PC without additional software.