Custom 5ESS if it is coming from a 5ESS.
If it is coming from a 4ESS good luck, it is not compatible with anything including AT&T's own quasi-standard TR 41459
Equipment configured for AT&T Custom 5ESS *may* work with the 4ESS but don't place any high stakes bets.
You'll only see the 4E if you are connecting directly to AT&T's LD network though, so that may help.
Best of luck