No, they are not all 617, and some small number do seem to be on other carriers. This batch does all seem to be in Massachusetts, though. As you point out, any advice you get from T-Mobile to change your email alias is likely pointless. You would most likely have to block e-mail to your base [phonenumber]@tmomail.net address.
Okay first relax on Tmobile, its really has nothing to do with them, and they really cant do anything. So that being said, here is whats happening.....
tienda-activa.com traces back to an IP Address (200.75.61.22) from Columbia to corporat200-7561022.sta.etb.net.co
While, artmotion.net traces back to an IP Address (84.22.33.6) from Albania.
NOW, tienda-activa.com IP Address (200.75.61.22) happens to be listed as mail server and dictionary attacker (hence the similar area phone calls).
Artmotion.net IP Address (84.22.33.6) happens to be a communications website for "Phone, sms, web hosting, and email" (haha GO FIGURE)
Tmobile cant really do much... VISIT and FILE COMPLAINTS at