Dropped connections and failed handset handovers are typical symptoms of a multi-cell installation with weak DECT sync source RSSI values.
To troubleshoot this, log in to the Web User Interface of each base station of the affected multi-cell and have a look at the DECT Chain and the Base Station Group. Check the DECT Sync Source RSSI values.
Make sure that the "DECT synchronization source" of each base station has an RSSI value of minimum -75dBm. Weaker RSSI values (e.g. -76dBm or less) can cause dropped connections and failing handovers.