Hello GeraInt,
thanks for your kind replies.
what do you mean by "pre-registered by him"? Is this only for logged in users? Is this user 'invited' to this event?
The website will be used by users which have an activity (doctors, car's mechanics, etc.). With the calendar (every user must see only his own events), they will have the possibility to record their customer's appointments (i.e. a person which have to fix his car) in a certain date and time and recording the customer phone number. Referring to this example, when the machine will be fixed, the mechanic will go to the calendar, will select the event and will click a button which will send an sms the customer.
How do you stop search engine robots or malicious users triggering LOTS of 'spam' SMS messages.
About the possibility of spam, i am thinking to separate the number in 3 different fields (nationa prefix - local prefix - number). I hope this will avoid the automatic spam but, honestly, i am not so sure.
Does this message include information about the logged in visitor?
The message will not contain informations about the logged in visitor. An example of sms will be "Hello, your car is ready. You can pick it up when you want."
Thanks again!