You can usually send anonymous texts via email. To accomplish this, you will typically have to know the cell phone provider the recipient uses as well as the email address the provider uses for sending messages to phones. Then, you can create a new email account with which the person is not familiar and use it to email a text message to the recipient's phone. On some phones, the message will appear just like a text message, and the person may not easily recognize that the message originated from an email address. On others, however, the email address may be prominently displayed, though the recipient may still have trouble figuring out who sent it.
The first step in sending an anonymous text message usually is learning the email address the recipient's cell phone provider uses for texts. You will have to add the recipient's phone number to it, and it will likely look something like this: firstname.lastname@example.org. You can probably find the appropriate email information on the cell phone provider's website or by using a search engine to look for it.
Once you have determined the right email address to which to send the anonymous text message, you will have to create a new email address with which to send the text message. There are many free email services you can use for this purpose, and you can use just about any name you want as part of the email address. It is important, however, to make sure using the email address for anonymous texts won't violate the email provider's terms of service.
With your new email address in hand, you can complete the process of sending an anonymous text. To do this, you simply open the page required to send an email from your new provider and type your text message into the message box. Then, you can add the email address that includes the recipient's phone number in the "Send To" field. You can make your message look more like a text if you do not fill in the "Subject" field. Once you've finished your message, you can just click "Send" to deliver the text to the recipient.
There are also some websites that advertise anonymous texting. You might choose to investigate some of these if you aren't interested in creating a new email address for the purpose of sending anonymous texts. It is important to keep in mind that the legalities of sending anonymous texts usually depend on the content of the text message. Laws regarding such matters will likely vary from jurisdiction to jurisdiction. In most places, however, using anonymous texts to harass or threaten others, trick others into revealing confidential information, or commit blackmail or extortion will likely cause you to run afoul of the law.