Customer had an ACS R1 - duel service from AT&T with caller ID. Both locations were getting and displaying caller ID. The ACS is at their place of business an the other is their home with single line caller ID phones. R1 died, replace with R6 now no caller id. Tried second R6 the same, both work at my office. Turns out when AT&T removes duel service they get caller ID on the R6. Turn duel service back on no caller ID. Central office has an older DMS XXX.