A landmark 1991 law, the Telephone Consumer Protection Act, or TCPA, originally protected consumers from telemarketing calls. Recently, it has been also applied to sending Text messages as well.
The law states that you must get prior express written consent before calling, sending a text message, or delivering a voice message to a customer’s mobile phone.
Simply put, don’t text people unless they give you permission to do so, it's illegal.
What is Consent?
Prior consent requires that your customer take an affirmative step, such as checking a box or sending a typed response.
For example, on a form you can not simply collect a phone number and start texting. You must include clear opt-in language to gain consent.
[ ] You agree to receive automated promotional messages. This agreement isn't a condition of purchase. Msg frequency varies, msg&data rates may apply. Reply STOP to stop, HELP for help.