I am installing a PC based 4-port voice mail on a 124i and I can't figure out how to set up 3 phones to ring on all lines and after 4 rings go to auto attendant. I have tried all sorts of combinations and nothing works. Anybody know the trick?

First things you have to do is put the 3 phones in their own unique ring group. Next, assign the trunks to ring that particular ring group. If no one answers a call, it must overflow to a secondary ring group, which would be your vm. All this programming is done under "trunk options". All your vm ports should be in ring group 16 btw.

I have a similar setup. Can I have delay answering during the day and immediate answering when the nite mode is turned on?

In night mode, DIL all trunks to the first port of vmail

Sure can. You can DIL the trunk, only I believe you enter the first port number of the vm group, not the group pilot number or under ring group programming in night mode, set it to ring group 16.