I need to enable SMS on a certain website. I guess the client wants to send (and receive?) SMS from the website. I'm guessing that one way to enable this feature is to set up an SMS gateway? How does this work? How much does it cost? How difficult is it to set up SMS gateway? (Or I'm totally misunderstanding this ...?)
You will need to use a SMS gateway. Some range from 4p to 10p per message. For example, www.clickatell.com or www.textover.com
2-Way SMS Gateway, From 0.014 EUR / SMS, High Quality, Delivery Reports, Free Trial, Easy setup and usage. Send and Recieve SMS. Let your customers to be the part of sophisticated trouble-free service using SMS. http://www.2-waysms.com
Some SMS gateways work on an e-mail address basis, so these aren't too hard to set up. You can often just send an email in a certain format to <phone number>@gateway.com. One example, hastily just Googled, is World Text.
You can try www.txt2way.com. It allows for sending and receiving text using email to SMS. I have been in contact with them for Business SMS. They write scripts and integration tools for websites.