3.3.3.2.1. Requirements 
- ISDN connections are only supported if Mediatrix 44xx gateways are used.
- ISDN voice connections are only supported for stations or trunks that are configured on the same OpenScape 4000 SoftGate.
- All gateways should be synchronized with a common clock. For instance, the gateways receive the clock pulse via a direct connection to the public network or via an internal ISDN S0 line (layer-0 connection) to another synchronized gateway, as shown in the sample scenario.