Hi Adam,
As of V15 SP4:
Please find Firmware http://support.yealink.com/documentFront/forwardToDocumentDetailPage?documentId=110
(found under w56 but include w52)
Go to "FXS/DECT"
Click "Add FXS/DECT"
Choose "Yealink" from the "Select Brand" drop down box
Choose "Yealink W52P" from the "Select...