Calls between systems using digital trunks, E1, E1R2, T1 ISDN and BRI, require an common clock signal. The IP Office will try to obtain this clock signal from an exchange through one of its digital trunks. This is done by setting the Clock Quality setting of that Line to Network. If there are multiple trunks to public exchanges, another trunk can be set as Fallback should the primary clock signal fail. Other trunks should be set as Unsuitable.