If you try adding an SMS number to an Account or Member with a cell carrier that is not listed, try entering your phone number at freecarrierlookup.com. For those using a mobile virtual network operator (MNVO) such as Tracfone, it should tell you the name of the actual carrier you are on, such as Verizon, and then you can select that name from the carrier list in your account profile and see if it works.
In some cases, however, the SMS gateway address is not the same as the address for the actual carrier. For example, even though Boost Mobile uses the Sprint network, <mobilenumber>@myboostmobile.com is used to send a text to someone through our system.
If the carrier name is not in the dropdown, whether for the above reason or if they are a new or unique carrier, contact Support with the information from the above tool, and we'll see about adding it.