MWI Not working with T20P

Hi

We have T20P's here and a swyx phone system. The MWI is working fine as far as when someone leaves a voicemail the phones pick it up showing a new vm, and then you can use the message button to retreive the VM. HOWEVR once you have listened to the vm and deleted it, the MWI is still lit showing that there are still new VM's.

Any ideas pls on how to get this to refresh?? If i logon to the backend of the individual phone and change the refresh time then hit confirm it realises that there is now VM's however it all starts again with a new VM. I have upgraded to the latest FW.

Thanks

Glen