i take it to mean that casey took caylee and left the grandparents home a month ago, cindy has been asking casey to let her see caylee for a month now. cindy does not consider caylee to be "missing" at this point. just that casey is keeping cindy from seeing caylee.
so she tells her she is calling the cops and they will order her to produce caylee.
casey is still trying to delay, most likely because she cant produce caylee for obvious reasons, and asks for one more day just like cindy has said she did all along, cindy has always said that when she finally met with casey and caylee wasnt there that casey told her she would get caylee tomorrow. just wait til tomorrow.
once the police are on the phone repeatedly, casey has to come up with something and says "listen mom, its not just that i wont let you see caylee, i havent even seen her in a month".
and then when cindy calls back the third time to 911, it truly is the first time she considers caylee a "missing person". i dont get the sense she is acting in that call, it sounds genuine to me, and there is a reasonable interpretation of events that supports that being the way it went down.