The rule regarding consent is that you must have express consent for conversational and informational messages or express written and logged consent for promotional/ marketing messages.
Depending on the type of message you are sending this consent can be obtained different ways, as outlined here: https://vimeo.com/ringcentraldevs/tcr-consent-requirements
Once you have consent for a type of message, that consent is valid to continue sending messages. For example, if you ask your customer if you can send them updates about your company it is valid until they opt out (unless there was a timeframe limitation in the agreement). You do not need to ask them each time for their permission, unless you are asking for permission to send a type of message other than previously agreed upon (eg do you want to receive promotions from our company?).
They key here is that you need consent before messaging them, so you cannot send them a text (unless they initiative a conversation by asking you a question via text) unless they have expressly stated that they want to receive text messages from you. Providing a phone number (resume, webform, paper form, etc) is not considered sufficient consent unless there is also language that states that by doing so they are agreeing to receive SMS messages from you. This is why you need specific language on any forms where you are collecting phone numbers for the purpose of SMS messaging.
