Went through AT&T's behavioral interviews twice over the last two years (once for a business sales role, once pivoting to try for a tech program manager position) so I have a decent sense of what they're looking for.
The behavioral rounds at AT&T are much more prominent than at pure tech companies. You can expect a dedicated behavioral round plus behavioral questions woven into the hiring manager interview. Probably 40-50% of the total interview time is behavioral.
The values they return to again and again:
Customer obsession. Seriously, almost every question I got had an implicit "and how did this impact the customer?" angle. This is a B2C consumer company at its core. Even for technical roles, they want to see that you think about end users.
Stakeholder alignment / cross-functional work. AT&T is a massive organization. They want people who can work across silos. Any example where you navigated disagreement or got multiple teams moving together is gold.
Driving through ambiguity. They use that exact phrase. Most of the behavioral prompts I got were STAR-format: "Tell me about a time when you had to make a decision without full information."
Questions I actually got (paraphrasing): Tell me about a time you had to push back on a stakeholder's request. Describe a situation where you missed a deadline. What happened and what did you do? Tell me about a project where the scope changed significantly mid-stream. Give an example of when you had to work with someone who had a very different communication style. Tell me about a time you improved a process.
What didn't work: Generic answers. The interviewers were experienced and they'd follow up with "and what specifically did you do" if you kept it vague. You need a real story with real specifics: a number, a timeline, an actual outcome.
The interviewer for my PM role specifically said they use a structured scorecard, so each question maps to a competency. You don't get to pick which competencies they score. Just be ready for the full range.