Can't I just set up an SMS auto-reply myself?

You can, and for a minority of you that'll be enough: a simple auto-reply sends "we'll call you back" and stops there. The difference starts right after: who answers when the customer writes "it's a leak, it's urgent?" — the auto-reply stays silent, Signal qualifies, rates the urgency 1 to 5, offers a slot and escalates if the score passes your threshold. There's also compliance: STOP within 24 h, the 8 AM–9 PM window, opt-out registry, timestamped proof — all things a homegrown setup forgets until dispute day. DIY is free; DIY doesn't recover the customer who wants an appointment at 9:15 PM.