I would imagine you would have to hook into a commercial SMS service. A quick google revealed there are plenty around and they all seem to have web based API's that you can access from custom applications.
Can SMS service provider do it all for me like PL/SQL development?
Can APEX hosting companies give me some guidance ?
Can you guide me in details, as I have never developed it before and also new to APEX ? I m from India.
Most commercial SMS service providers have an email-to-SMS service, therefore very simple to integrate.
The solution is simple and isn't Apex specific. You can use it with any application: forms, reports, Apex, etc.
All you do is use an SMS to email provider (lots can be found on Google) so when your Apex application to sends an email to them the provider will turn it into an SMS.
I've used one of these providers in the past and it works well, is inexpensive and delivery is fast. You register each email address you want to use with the provider and then store the email address in your database. When you subsequently send an email the provider takes care of the rest.
Andy - snap!