After some trial and error - this is what seems to work for me:
frappe.ui.form.on("Sales Order", "on_submit", function(frm){
frappe.call({
method: "frappe.core.doctype.sms_settings.sms_settings.send_sms",
args: {
receiver_list: [frm.doc.contact_mobile],
msg: "Your Order "+[frm.doc.name]+" for the amount $"+[frm.doc.grand_total]+" has been received",
},
})
});