In the United States, consumers are protected by the Telephone Consumer Protection Act (TCPA). The TCPA states that you must have express consent given from every single person you send a marketing SMS to. That means if you import a list, you must have express written (or digital) consent from every single person on the list.
The following does not constitute legal advice. Please contact a lawyer if you have specific questions about complying with the TCPA.
If someone used to be a customer, or fills in a causal sign in form, they have not given their consent. A previous business relationship does not imply consent in the TCPA.
As well as obtaining consent, you also need to include a simple opt out function, and every SMS needs to contain the details of the sender. These details can be on a webpage linked in the message.
For each unauthorised SMS you send that does not comply with the above regulations, you can be fined up to $1500 USD. That racks up very quickly.
GymLeads helps you stay on the right side of the TCPA by providing an auto opt-out mechanism, and including a link with your details in every message. It is your responsibility to ensure you have express consent for every lead you send marketing messages to.