If they can receive e-mail, you can use Internet Tool Kit from NI or direct to port 25 of the mail server (to send them).
Another way is to command a faxmodem to phone them and play a sound message.
The most difficult to implement is the option of sending SMS for cell phones(If you want to do it free, you have to use some web sites to send them. Otherwise, there are some plug-ins to do the job - like those for ICQ).