SMSPlug

What this module does is realy simple, after installing in /sites/all you can
activate the module and do some settings: numbers of sms per user and
month, set the originator, userkey, password and what should all be
loged. That's all for the admin part.

The module generates a block which can be set left, right or by the
content.

The user gets a form where he can search for a user (autocomplete) or
enter a number directly. That autocomplete works there must be a field
within the profile called "profile_handynumber". After the number is
selected or entered, the user can start to enter message and after that
send it. You will be notified if the sms was send successful or not.