T46G MWI not clearing

Hi,
we have a few T46G IP Telephones here connected to a Fritzbox 6360.
Everything was working fine until our provider (Vodafone/Kabeldeutschland)
rolled out a firmware upgrade to the modem (Fritzbox 6360) yesterday night.

Since then the Message Waiting Indicator for voice mail is constantly blinking - even if I have deleted any new messages on the fritzbox.
The voice mail button is correctly connecting to the fritzbox voice mail. It is working alright - but the Message indicator is constantly on....

(fritbox has been reset and newly configured- also to no avail).

Any Ideas?

p.s. we use the latest firmware on our T46G phones 28.80.0.130