When I want to quickly contact my online students, I use Remind.com. It allows you to send a text without knowing their phone numbers. Additionally you can set a time for it to be sent. You can add a link and/or upload a document. If you choose, you can also text individual students. All of the texts are recorded and kept as long as you keep the class.
Above is a video giving you a great overview and here are several videos explaining how to use different parts of it.
If you use Blackboard, the video below shows you how to embed Remind texts into a Blackboard page.