Just wrapped the full loop for a senior SWE / tech lead adjacent role at AT&T's technology and operations division. The system design round deserves its own write-up because it's different from what most people prep for.
First: AT&T doesn't use L5 as a label. Their bands are something like SWE I / II / III and then Principal. The role I interviewed for was their SWE III level, which from comp and scope is roughly equivalent to senior / L5 at Big Tech.
The system design format: One hour, one interviewer (a principal engineer). He opened by saying he wanted to discuss a problem relevant to what the team actually does. He gave me a design problem involving a large-scale billing event pipeline. Think: millions of events per minute, subscribers generating usage data across 5G and fiber, needing real-time aggregation for billing and anomaly detection.
This wasn't a generic "design Twitter" type question. It was domain-specific and they expected me to at least ask smart questions about the telecom context, not pretend I'm designing for a web startup.
What he cared about most: Fault tolerance and exactly-once delivery (billing errors are really bad for a carrier) How I'd handle backpressure when consumers fall behind Data partitioning strategy and why Trade-offs on consistency vs. availability, and I had to be specific about which parts of the pipeline could tolerate eventual consistency
I used Kafka for the event backbone, Flink for stream processing, and Postgres + a columnar store for the aggregation layer. He challenged every major choice. Not in an adversarial way, more like an architect reviewing an actual design.
What I wish I'd prepared more: Telecom-specific data volumes and SLA expectations. I knew distributed systems but I kept having to say "I'm not sure what typical AT&T-scale numbers look like" which was fine, but knowing rough orders of magnitude (like: they have 200+ million subscribers, events per session, etc.) would have let me anchor the design better.
I got through to an offer. System design felt like the heaviest weight in the decision.