Hi Nigel,
Unfortunately there's no way for the 8x8 Phone App itself to control volume within Teams Mobile, which is where the control and presentation of actual calls takes place. From there the Teams Mobile app relies on the phone manufacturer's implementation of the OS (Android) for ultimate volume control.
I found that Teams Mobile doesn't currently allow you to change the single fairly weak and unobtrusive ring tone, so that does not appear to be a solution. There are several complaints about this on Microsoft's forum.
I dug through my own Android phone (Samsung S21 FE 5G) for ring Settings and to see if I could potentially even replace the audio file Microsoft implemented, and unfortunately could not find anything that could help you.
So my apologies: I'm afraid that there's nothing I can offer you to resolve this.
I know that it won't be of immediate help, but I suggest giving Microsoft feedback about this omission. If enough customers complain about this, Microsoft might be more inclined to fix it.