Open the Voicemail Reports, go to view voicemails Tab. The system will show the voicemail list.
When you try to Export the data by clicking on Export Data button and selecting the format,It will not export the data instead it will insert "Export to xls, Export csv and Export to txt" buttons irrespective of the format selected. And these buttons are also non responsive which results in the failure of data export to different format.
Note: Works fine in FreePBX 13