Switching To Cox from AT&T----Phone Service Concern

I’m planning to switch from AT&T landline to Cox for phone service (I'd also get Cox internet and TV). I don’t want to port my phone number prior to installation in case the install doesn’t go well or doesn’t happen at all. But from the time of the install until the number is ported (however long that takes), it seems that I won’t have any access to the AT&T number (I don’t have AT&T voice mail), since Cox will disconnect the AT&T lead and connect their own. I can’t tell how to connect an RJ-11 cable to the NID to keep one hard-wired phone working with AT&T until the number is ported to the Cox line; when I open the NID, I don’t see a jack. I’d appreciate any suggestions. Thanks.