Resolving Auto-Reply SMS Failed Errors on Dual SIM Android Phones

Dual SIM Android phone showing MacroDroid Send SMS action with SIM 1 and SIM 2 selection and the default message SIM setting

Auto-reply SMS that works on a single-SIM phone and fails on a dual-SIM phone is almost always a SIM selection problem, not a MacroDroid bug. This guide starts from the System Log error and a one-SIM-at-a-time send test, then pins the default message SIM on Pixel, One UI, MIUI/HyperOS, and ColorOS, sets the SIM explicitly in the macro, and verifies delivery in both directions on both SIMs.