SIM800L - Play wave file from Arduino DAC to microphone

Official Hardware Retired GSM Shield
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Telephone quality audio is sampled at 8 kHz, and each sample is 8 bits.

So every second of audio is 64 kilobits, or 8 kilobytes. Ten seconds is 80 kB.

On my microcontroller, I use the SIM cell tower ID to get the device's longitude and latitude, and then I compose a sentence like:

I am located at 51.5007° north and 0.1246° west.

I then use a text-to-speech engine to convert this sentence into an 8-Bit mono 8kHz wave file. The wave file will be about 10 seconds long, for a total of 80 kilobytes. I store these 80 kilobytes in a global array:

char unsigned samples[8000ul * 10ul];  /* 10 seconds of 8-Bit at 8 kHz */

So next I wait for a phone call to come in on the SIM800L, and my microcontroller sees the "RINGING" message come in. My microcontroller answers the call. I have DAC pin on my microcontroller connected to the "MIC" pin on the SIM800L.

So here's what I'm thinking I'll do:

(Step 1) Set a global pointer to the beginning of the array:

char unsigned const *ps;

void PlayAudio(void)
{
    ps = samples;
}

(Step 2) Enable an timer interrupt that fires at a frequency of 8 kHz (i.e. once every 125 microseconds)

char unsigned const *ps;

void PlayAudio(void)
{
    ps = samples;
    timerAlarm(mytimer, 125u, true, false);
}

(Step 3) Inside the interrupt routine, write the current sample to the DAC pin:

void InterruptRoutine(void)
{
    // Write an 8-Bit audio sample as a
    // voltage to the DAC output pin
    dacWrite( DAC_CH1,  *ps++ );

    // The next line brings the pointer back to
    // the beginning if it goes past the end
    // (so the audio repeats forever)
    if ( ps >= samples[sizeof samples] ) ps = samples;
}

I haven't coded this yet but I think it should work. The output voltage range of the DAC might not match the input voltage range of the MIC pin, but I can use a resistor ladder to match them.

Has anyone already tried this and gotten it working? Anyone got any thoughts to share?

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