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The problem with the DDCB/MDFB is that it is designed to ring onto the digital/IP handsets as you are only able to ring a DN or page group.
So by making the doorphone ring to your SLT 666 that will be the only destination ringing unless you put a SDN key of 666 on your digital desk phone and set it to ring immediatly.
The downside of that is everytime 666 is called it will also ring on your desk phone as well.
You could set the SDN to delay1 ringing so it rings the SLT 666 and just flashes on the SDN key for say 12 seconds before it starts to ring but you can press the SDN at anytime to answer the call. Just make sure the doorphone ring time is greater than the delay1 timer on the desk phone.
Hope that makes sense as it has always been a limitation of the DDCB/MDFB that it couldn't ring a MCG or a hunt group to include SLT handsets.
The other option is to go for a different doorphone setup that is connected to a spare SLT or a PSTN line port that way you can make the line port ring to a MCG group of DKT/IPT/SLT handsets, or the SLT type can go off hook and ring to a MCG as well.
Dalas Delta doorphones are one example of that type instead of the Toshiba ones.
At least you have learnt about the doorphone different configuration/limitations so you won't make the same mistake next time.
Have fun
