The easiest option in this case – is the installation of software from the mobile operators with the ability to send SMS. Most often, it comes bundled with software designed to connect to the Internet. You just send messages from your mobile using unlimited SIM card. However, not the most fast and convenient way. Moreover, almost always there are limits to the number of free SMS.
Make free SMS sending through the Internet through the websites of mobile operators. There is also a limit on the number of messages sent from a single IP address. In addition, you will have to enter a security code and that the extra difficulties. Sometimes on these sites you can find stocks by free mass distribution, but they are rare.
The most convenient way is to use special offers from the firms which are engaged in mobile marketing. You can ask them test the newsletter on few numbers (usually within 100) absolutely for free. If you have a small number of customers, this method might work for you, but for long term it is not suitable.
Instead of the free ones, it is better to use the paid services of specialized firms. This will save a lot of effort, time and money, and provide you a more reliable delivery system.