All Siemens Gigaset VoIP DECT handsets support 2 VoIP and 1 PSTN / Landline call simultaneously.
Now all of the base stations within the range, have the ability to perform automatic failover to your landline. Quite simply, if for any reason your VoIP service or PBX fails causing your internet connection to go down, the Siemens Gigaset will automatically send calls over the landline connection plugged into the base station instead. No chance of missing an important call.
Just configure your base station to fail over to PSTN on the web interface.