I imagine you have a server at your house, where Gammu must be running.
By being in your home the Gammu, and your home probably use a home link with the internet, you have a dynamic IP address in your residence.
Therefore, in the first place, it is necessary to register a dynamic DNS pointing to your home. Use a service like No-Ip or Dyndns. You will have to install a program on your home computer, preferably on the same computer as Gammu (since you probably won’t turn it off).
Once this is done, you need to configure your router to direct ports to your computer with Gammu. See in the documentation of the software that communicates with Gammu which network ports are used. Note that this application acts as a server, taking commands and passing them to Gammu to run. If you do not have it, you will need one. I don’t know any, so I won’t recommend it, but Google quotes a few. Otherwise, build your.
There are classes that communicate PHP with Gammu, but PHP needs to run on the same computer as Gammu. Therefore, a solution would be to also install a webserver on the computer with Gammu. On your hosting server, you would send commands to the Gammu server, and this in turn would send commands to Gammu.
Whether you use a PHP communication code with Gammu or not, you will need an application on your home server to interface with Gammu and your remote webserver, as well as the settings on the router and dynamic IP.
His answer by the title of the question is considered correct, but I think he wants you to execute a command in Gammu as if it were a "service".
– Guilherme Nascimento