Definitely not the case. Think of it as T-Mobile's version of Google Voice using your T-Mobile number--or you can pay a monthly fee to have a totally different phone number that lives only on the DIGITS app. In fact, I can use the DIGITS app on my tablet (wifi-only), and I can set it up to run all four lines in my account. If I wanted to, that is. I just enable DIGITS on each phone (you need to keep your T-Mobile SIM card in to receive the 2FA text message), then I insert local SIM cards in each phone, and they're good to go. When I swap back to our T-Mobile SIMs, I turn off DIGITS on each phone.