You should consult the phone manufacturer's documentation or their support offering (wiki, forum, etc.). Some phones will require URL dialing to be enabled first. However once done, then the format will be the same for all. However, this may also expose the phones to other folks using various tools to try and hack systems and phones as they will be using the same format. This may then allow so called "ghost calls" to start showing up. This is when phones start ringing, an unknown caller-ID may show up and when the phone is answered, there will be no one on the other end. They will be relentless about attacking afterwards/