Is there any known issues with Plantronics M12 Amp / H-series headsets and the Telrad 128?

What problem are you having, I don't have a compatibility ref for the Plantronics but I am not aware of any problems.

What phone are you using it on?
you should have a cat# on the bottom of your phone.
cat# = catelog number.

I just had a Plantronics vendor tell me I may need to use an AC adapter because there was a "wake up" issue with the Telrad Digital.

I have no idea what he is talking about. Never heard of a "wake up" issue.

Just put batteries in the M12 you will be fine. No issues I have used M10's and M12 for years. Batteries and set to 5 it will work.

Very few people use the optional AC adapter. You really don't need it and it just ends up being one more cord to get tangled up at you desk. Two AA batteries are all you need. If you still have problems feel free to email me.