Finished the T-Mobile senior SWE loop in April. Posting specifics because I searched for this before my interview and found almost nothing useful.
The system design round was 60 minutes, one interviewer. The prompt I got was roughly: design a real-time notifications service that can fan out to millions of mobile subscribers with low latency. Pretty classic telecom-flavored design problem, but they cared a lot about specifics that you wouldn't necessarily emphasize at, say, a pure SaaS company.
What they actually probed on: Scale: T-Mobile has ~120M subscribers. How does your design handle a burst like a national weather alert? They wanted Kafka or a similar durable queue, not polling. Durability vs. latency tradeoffs. I mentioned eventual consistency and the interviewer pushed hard: what happens if a message is delivered twice? What if it's dropped? Delivery channels: push notification vs. SMS vs. in-app. They wanted acknowledgment handling. Database choice: they were fine with PostgreSQL but wanted to hear me justify it. Column store vs. row store came up briefly.
Format: screen share in Google Meet. No collaborative whiteboard tool, just a blank Google Doc I had to diagram in text. Bit awkward. Bring a tablet if you can.
They did NOT ask a LeetCode-style coding problem in the system design round. That's separate (more on that in another thread).
Level calibration: this was for a senior SWE role, Bellevue-based. The expectations felt comparable to what I'd call a Staff-minus at a smaller company, but lower bar than FAANG L5 system design. They seemed more interested in whether you understood the telecom context than whether you'd read every distributed systems paper.
Timeline: phone screen to offer was 6 weeks. Debrief to offer took about 10 days after the onsite, which felt long but HR confirmed it's normal for them.
Happy to answer questions about specific rounds.