Iridium allows to send free text messages (SMS) to Iridium subscribers.
Via the Web: You can send a FREE text message to an Iridium subscriber from the Iridium Send SMS page.
Via email: Create a new email message addressed to 8816XXXXXXXX@msg.iridium.com (where 8816XXXXXXXX is recipient's Iridium number), enter your message in the body of the email and send your email. Any text placed within the subject line will not transmit.
Please note that receiving of free messages has a lower priority compared to paid messages. Also, there is a limit of 150 free messages that can be received monthly by an Iridium subscriber. More information