playSMS is a free and open source SMS management software. It allows to run a self hosted SMS Gateway, using a connected Phone (or USB-GSM Modem like Huawei E173) for sending SMS. Messages are sent via a http-post method. (i.e. http(s)://your.playsms.url/index.php?app=ws&u=“username”&h=“token”&op=pv&to=“mobilenumber”&msg=“message”)
Parameters to provide:
URL
Username
Token
mobile phone
Message
HomePage : https://playsms.org
Great playSMS Installation Guide : https://www.howtoforge.com/tutorial/how-to-install-raspberry-pi-sms-server/