I do not have a lot of contacts, but I would like to build a quality base for mailings. 
 

The lack of a base of contacts prevents the launch of SMS marketing and obtaining additional sales. But do not despair, there are available ways to form a base of numbers of your audience, for example, you can use the services of the company https://sempico.solutions/ . But the most important thing in this case, that you should remember not to chase easy money and not to use illegal ways. That may be considered as spam and illegal use of personal data. You should always keep this in mind.
 

There are some simple ways, such as a website registration form, a purchase form, registering for free services, or a direct offer to sign up for the newsletter. Using such simple and legitimate ways will help you.

Thankfully, I've never had problems with it, but I can tell you it took me a while to discover a solution to help me with waivers. If you're interested, you can check out this link https://www.pandawaiver.com/, and with this software, it's not hard for me to create, customize, and e-sign digital waivers, so it's actually convenient.