Just go through all the groups... 500-518 unless they renumbered those for 2 digit as well. It won't let you turn it on for a group you're not a member of anyway.
Only using one phone at this stage. It reacted to 53-500 and I pressed 1 and got a confirmation msg that I will now receive group calls. Did not change anything. Incoming calls still seemingly ring to EXT5 or ID 25.
Phone shows "21: Reception" and the date below it when plugged into EXT1. Same for EXT2, 3 and 4 with just a change in the ID to 22, 23 and 24 respectively and of course the name we gave the (unused) stations.
Still 100% in the dark on how to get EXT 1-4 ringing again.
