Is it possible to send a message to a fax from iPhone/iPad

Is it possible to send a message to a fax receiver from an iPhone/iPad application? Or do I need to use some online services? If so, which should I use?

The iPhone/iPad can not be used to send faxes in a traditional manner, unfortunately, except in the case that it is used as a simple modem by a computer (which I don't actually think is possible as it isn't a CDC-class device.) There might be some applications available that are frontends to web services, however.

Barring that, you can send faxes online using http://www.efax.com, but there are a lot more free fax providers if you search.