You can send text messages to US cellular phone numbers from any online computer.
How to do it? Simple. From your email client send the message to [cellphonenumber]@teleflip.com, where "cellphonenumber" is the phone number you want to send the text SMS message.
Example: Your target cell phone is 444.555.9999. So you send the email message to firstname.lastname@example.org and the cell phone will receive it as a SMS.
Teleflip is free.