Reading DTMF tones from a Motorola Razr V3 with a Z80 system

This is a demonstration of a bonus feature I built into my Z80 rotary phone system. Basically, I can wirelessly enter data with a Motorola Razr V3.
How does this work? Basically, the phone system has an inductor pickup coil that can detect low frequency (audio range) radio signals. When I push a number key on the Razr, not only does it make an audible beep sound from the earpiece, but the earpiece also emits a small radio signal at the same frequencies, which I can then detect with the pickup coil.
The inductor pickup coil is input to an op amp circuit to amplify the signal, which is then input to an MT8870 DTMF Decoder chip. The output of the decoder chip is connected to channel B of the Z80 PIO. When the PIO receives data from the MT8870, it then interrupts the Z80 CPU, which reads in the decoded digit. The Z80 then sends the digit to the number display.
I plan to leverage this feature a little more in the future. Why would I need this feature? Well I don't really, it's just an excuse to play around with a Motorola Razr and a Z80 at the same time.