Why isn't Verizion Message+ working on my old phone?

[AC Question]

Okay so here's my problem. I have Verizion Message+ on my main phone (HTC One M8) but I want to be able to still message through my old phone (Samsung Galaxy Stellar) with Verizion Message+. My old phone, when I switched phones, had calling and texting disabled, so basically its like an iPod in the sense that it can only do stuff over wifi (apps, internet). I installed it on my old phone to use it, but it reads my old texts and not my new ones from my HTC One M8. My old phone has the same number as my new phone if that helps.

Thanks!