It is not legally required to provide customers with instructions on how to opt out. But it is a very strongly recommended best practice to provide customers with this information. If you don’t do so, it is possible that people will opt out with their own legally sufficient language that is not recognized by your system, leading you to continue to send messages or make calls which now violate the Telephone Consumer Protection Act (TCPA). You need to be able to recognize and honor opt-outs, so always include a simple opt-out instruction (“Reply STOP to Opt-Out” for example).