Turn a led on, when a button is pressed on a Yealink SIP-T42S

Hi, I deliver VoIP services to my customers and i use 90% of the time the Yealink SIP phones. The problem in 50% of the cases is that when the client is turning on their voicemail, they can't see it anyway if it is turned on yes or no. I know it is possible to use something with XML and a webserver to create a connection with the phone which can enable / turn on a led to indicate a button is pressed with a specific command, like enable voicemail. When the client presses an other button, it should disable the led and execute a specific command. I need you to get this done, i can do all the testing, but i have no clue where to start. I know a competitor has made something like this and it where a few files on a webserver which he was calling from an action programmed on a button of the phone, so it can't be that difficult.