Comment by tamimio

Building a homemade BTS (Base Transceiver Station) is easy. I remember making one back in 2011 with a USRP SDR. Nowadays, you can even create a 5G network, not just LTE. There’s plenty of good open-source software available. Paired with an SDR, you are good to go. The rest is just some scripting to automate some tasks, probably how they flooded the SMS.