Hey, button you created wasn't showing up for me as well. Image link you used was not working, you did not define any color for the button and you might also positioned it outside of your screen.
I created example for you here.
Also don't forget to assign permission or the button is not going to show up after reloading plugin.
{
"phone": {
"uiType": "Overlay",
"panelType": "button",
"OffsetMin": "190 20",
"OffsetMax": "230 75",
"text": null,
"panelColor": "0 0 0 0.7",
"panelChatCmd": "phone",
"imageUrl": "https://static-mpm-optimized.s3.eu-west-2.amazonaws.com/0c67f666-9a63-44a6-bd1c-bba66bfe3ca7_large.png",
"imgAnchorMin": null,
"imgAnchorMax": null,
"onCmdHide": "false"
}
}