If you are trying to call a regular telephone number using the "internet" setting, the call will fail. The "internet" setting is specifically for SIP URI calling, an address which looks a lot like an email address. For an example of SIP URI calling, enter
test@rf.com in your RF Dialer and you'll hear an explanation of the service, and will be able to record a message that will be played back to you (so that you know what you sound like to others).
To dial a regular phone number using the RF Dialer (to a call overseas, for example) you need to register a calling service (either a VoIP service provider or an office PBX) with RF.com. Once you register your VoIP service, it will appear as a choice on the pulldown menu, and you would be able to make VoIP calls to regular phone numbers simply by selecting your service and dialing.
For more detailed information, check out these two entries in our Frequently Asked Questions:
Using RingFree without a service provider/PBX Using RingFree with my service provider