As pointed out by Engadget, the idea behind this tax dates back to the 90s where the US government implemented a Public Purpose Program. This program put a surcharge on all phone users, in which that additional money would be used to support other programs aimed at helping low income earners.
However given that voice calls are made less these days, thanks to users texting each other and/or leaving voice notes, revenues for the program have fallen by about a third, and conversely the budget for supporting low income earners has increased by 50%. By taxing text messages and classifying it under the same category as voice calls, it could help raise $44.5 million a year.
Carriers are reportedly not too thrilled by this program, but it should be noted that it is not set in stone yet as the FCC will vote on the measure come January 10, 2019. It should also be noted that this only applies to text messages, like SMS. Since other services like iMessage, WhatsApp, Messenger, LINE, Telegram, and etc. are sent over the internet, they won’t be taxed.