Not so. Although the box on their website is labeled "contact" you can simply type in a phone number and it'll work.

Keep in mind though that sending SMS is not supported to most countries outside of the US/Canada. You can receive, but not send/reply (including replying via email). Also MMS does NOT work (send or receive), and that includes group SMS messages - and the sender gets no indication that the message has failed. I ended up moving from voip.ms to Google Voice simply because I was missing too many messages due to them being sent to a group.

I've used the same one and highly recommend it. You do need to give up your credential to the author though in order to get push messages, which is needed for it to work real-time.