It worked - I have always used unlocked phones and you do get voicemail notifications without doing anything further. (There are still possibilities of issues if you bought European model it won't have US Carrier's APN Settings etc. but that's hardly reasonable to expect.) He wanted T-Mobile VoiceMail app to be bundled with it and also WiFi calling - both are carrier specific features which would have worked fine if he would have bought the right phone -T-Mobile HTC One. Expecting that to work on an unlocked, carrier unspecific phone designed to be LCD on as many GSM carriers is nothing but unreasonable.