Trouble with in-band tones (DTMF) when using MFI hearing device

iPhone 12

iOS 16.6

I use MFI hearing aides.  Dialing a telephone number works just fine.  However when connected through to a voice response system which then prompts me to input an extension, the system does not understand the subsequent digits dialed (in-band DTMF).  

Note that if I turn off my MFI hearing aides and repeat this process, everything works fine.  The voice response system accurately interprets the extension dialed.