You need to customized message like
frappe.ui.form.on("Sales Invoice", "on_submit", function(frm, cdt, cdn) {
var message = 'Invoice Number is ' + frm.doc.name;
frappe.call({
method: "frappe.core.doctype.sms_settings.sms_settings.send_sms",
args: {
receiver_list: [frm.doc.contact_number],
msg: message,
},
callback: function(r) {
if (r.exc) {
msgprint(r.exc);
return;
}
}
});
})