Ever since I upgraded my Dualphone 3088 from v109 to v115 it has become practically unusable.

It keeps getting disconnected from the base station and the skype call quality is atrocious. Voice keeps breaking, cracking and stretching. Not even a single word is audible, although the other side can still hear me.
Not even skype test service (echo123) is audible.

Dualphone.net support suggested resetting the phone and ever since I have done that the option for selecting firmware v109 form the handset has disappeared and I think I am stuck with v115

Is there anyway to reset the system back to v109 ?

Thanks.