I'm wary of putting phone numbers or email through a third party. "Free" often means someone's collecting data for marketing. I looked on the site and found a privacy policy pertaining to their newest version of the service. Here's a couple excerpts:
Teleflip may sell, for its own account, or otherwise release, your personal information to related third parties (such as our partners, bankers and advertisers) in order for them to notify you of products and/or services that may be of interest to you, or for any other lawful purpose;
Teleflip may use your personal information to advise you of new or additional services offered by Teleflip and/or its Affiliates; provided that all such correspondence shall alert Users of Opt-Out procedures in order to stop receiving such correspondence.
You may be safe with the version described in this forum, but I'd rather avoid the third party and use the wireless carrier's domain address.