I switched from AT&T to T-Mobile primarily because the service from AT&T had degraded to such a low level that it was almost unusable.
While T-Mobile service isn't perfect, it is far better. Outdoors I get a signal anywhere from San Francisco to the South Bay (since you have a San Bruno location tag). There are places where indoor coverage is weak.
I don't use much data and my phone is too old to get high speed data (no hspa+ or lte).--
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