Longstanding audio problem Q2686/87 H/G

Executing a “AT+WTONE=…” from an ADL application does not always result in the tone actually playing. Same when trying to use the ADL audio service to do the same thing.

In my specific case, I have the application send an SMS, wait for a certain SMS as response, and play the tone to indicate success.
On UART1 I only use RX and TX (no handshaking), and DTR is kept high. UART2 and USB not open.
Verifying with an oscilloscope in the no actual tone case, I can see that a base voltage level is applied to the speaker, but no tone is being generated. Only sometimes does the tone actually play and the oscilloscope shows the proper audio waveform (and the base voltage level).

The only workaround I have found to this problem is to set DTR low just before AT+WTONE and set it high again after. DTR is kept normally high to reduce power consumption.

I have reported this to Wavecom previously, but they only claim that this DTR usage can’t have anything to do with it and ignore the problem of no tone generation.

Tested on Q2686H with 6.61-6.63c and Q2687G with R7.2 firmware.

Tone generation works OK when the module is in a higher power consumption mode due to USB being in use or voice call going on.

Has anyone else noticed any problems with tone generation?