HL6528 audio levels


I have 3 x HL6528 modules; 2 with V3 firmware and 2 with V8.10 firmware. I am using the analogue audio interface with a gain stage of 2 added to SPKR_N/SPKR_P.
About a year ago I measured/recorded the audio level from the gain stage when receiving DTMF tones from my landline phone (IP phone).
I also did this test on a Huawei module.

I have performed the tests again this week and the Huawei module is giving similar results to my original test but the HL6528 module levels are significantly reduced (but repeatable) and the levels vary from module to module. I am using the same network/SIM (EE) and audio parameters.

My original tests measured a level of about 900-1000mv (pk-pk) but now I am measuring 350mv on the V3 module, 200mv on a V8.10 module and 500mv on the other V8.10 module.

We are in the process of checking for hardware differences. However, I am also wondering if anything else can influence the audio level such as the module itself, network cell or the codec selected by the network ?

Thanks in advance.

We have checked the hardware and there are no differences (gain stage is the same). Therefore it seems that the audio level differences are due to the module or network. Any ideas on why this is ?