I was surprised, but my new Verizon HTC One M9 have accepted T-Mobile prepaid SIM card. It does display "Unknown SIM" message and annoying icon on top of the screen, but let me place and receive calls and messages. I'm going to try data connection next. It looks like I will not need to exercise free replacement option due to switch to T-Mobile after all. I thought that only i-devices are not SIM-locked on Verizon - so did it change recently? Any ideas about removing annoying "unknown SIM" icon will be appreciated.
xda-developers