Remind (formerly Remind101) might be just the service you are looking for.
Remind allows you to set up classes and share a code with anyone who you want to put in that class. This might be students, parents, or other educators. People can sign up using their phones by sending a text with the code you share with them to your Remind number which is set up when you join. Alternatively, they can sign up online visiting a link that you share with them and enter the code there.
Once people join your class you can begin sending messages. You can include an attachment with your message as well. Your message can be a text or a voice recording. You can send messages from any device using the online version or the handy Remind app. You can schedule messages to be send in advance or you can send them at the moment you want them delivered. Keep in mind that people cannot reply to your message.
There are so many uses for Remind. I work with teachers in seven different buildings. I use Remind to push out messages about 30 minutes in advance to let them know when I will be in their building. Administrators can use Remind to communicate with their teachers regarding meetings. Teachers can use it to remind students about important things such as when projects are due. Teachers can use Remind to alert parents about important things happening at school. Remind is a and incredibly useful and versatile tool and it is FREE!