Cell phone service uses different radio frequencies (or "channels") for different functions. One channel is a control channel that is used to ring your phone, notify you that you have a new voicemail, etc. The control channel is also used for carrying text messages. The actual voice call is carried on a separate channel. Data is usually another channel.

So, it's possible to block just the voice channel and allow text messages by just turning off the voice channel, but that would seem to defeat the purpose of blocking remote bomb activation. A text message could just as easily trigger a cell phone-activated bomb as a voice call.

Sounds more like the voice channels were simply overwhelmed with too many calls, making voice calls impossible but still allowing text messages and data traffic through on the other channels.