Default on this is a call key on key 1 .
If you have all the same type of sets like 24 B then you can globally program a set type in mmc 723. Default ringing is to group 500 so change that or add the other phones to the group

Because you know you need to change the battery I wouldn't program to much back as you will have to do it again when you get the new battery.Me I use a cordless telephone replacement battery as long as its 3.6V