The CMX867A's “high-quality” DTMF decoder, for industrial control and security systems, telephone telemetry and advanced payphones offer high immunity to falsing on voice. A high-speed serial bus, ...

When a DTMF tone is received, a NodeMCU connected to the decoder calls an IFTTT job to echo the key to [WTH]’s phone as an SMS message.

His DTMF decoder box uses a CM8870 DTMF decoder chip, which you might assume is all you need to get the job done.