SMS messages have an almost 100% open rate. Not only that, but they usually get read immediately after sending them. This is why they can be an extremely powerful part of your marketing toolset
Before we even get into this, here's my disclaimer: SMS rules are constantly changing and you should read up on the latest rules before you try any of this. Basically, don't send msgs to people who do not want to receive them.
The first thing you need to do to setup SMS broadcasting is setup 2 accounts:
Deposit the minimum amount into Twilio ($20 i think) and buy a phone number in any area code you want.
Grab the Account SID and AUTH Token from your account Dashboard. You will need these for the next step.
You are probably going to at least need the 'Standard' Plan which allows you to send to 10,000 contacts.
When you outgrow that plan, you can go to the Premium plan and have unlimited contacts.
Follow the instructions on the site to link your Twilio account.
Once you have it setup you can go into the contacts section and do an import. It will accept mobile phone numbers in txt format, one per line.
Make sure you go into the services section and add the subscription service. This will make it so people can unsubscribe automatically when they respond back with 'STOP'.
If you run into any trouble telerivet has a great support staff available to answer your questions.
Telerivet can also be setup so people can 'TEXT SOMETHING to 555-555-5555' and subscribe to the list that way.
They also have an extensive API, so if you are developing some kind of team site, it can be integrated in a way that allows you to automatically add new members to the sms blast as they join your team.