Error when assigning SANGOMA DECT Phones on EPM

I am having the most bizarre error that i just cant get my head around.

We have bought several Sangoma DECT Phones. we bought 12 in total. each with its own base station as it is being used in several areas of the building. 

Now we have been trying to add these phones via the commercial EPM module. We have followed the instructions to the T.

We added the base stations, named them appropriately with the appropriate profile assigned. When it comes to the actual handsets, you need to join them to the base station. First you add the extension. then you edit the extension (in Extension Mapping) by adding the IPEI code to the appropriate Base station. Here is where we get the error:

Selected account[ 'account1' ] and mac ['' ] is already associated with extension[ '2130' ].
Please select another account if you want to associate with same mac address.

now i can assure you that this is incorrect. The extension that we working on is: 2126. 

We have cleared the browser cache, we re-installed the EPM, restarted the server, used a different computer and we just can get this resolved.

Hope you can help