diff --git a/lib/handler-callback-server.js b/lib/handler-callback-server.js
index a04312b..0688c58 100644
--- a/lib/handler-callback-server.js
+++ b/lib/handler-callback-server.js
@@ -30,6 +30,10 @@ function apikeyChecker(req, res, next) {
 
 function onIncomingSms(req, res) {
     res.end('OK');
+
+    if (!req.query.number) return;
+    if (req.query.number.indexOf('+') !== 0) return;
+    
     const numberWithSuffix = req.query.number + (config.number_suffix || '');
 
     partnerLastSeen.set(req.query.number, req.query.modem);
@@ -46,6 +50,7 @@ app.use(function(req, res, next) {
     logger.verbose('HANDLER-CALLBACK-SERVER: Incoming http request', { ip: req.ip, url: req.url });
     next();
 })
+
 app.use('/apikey/:apikey', apikeyChecker);
 app.get('/apikey/:apikey/on-sms', onIncomingSms);
 app.get('/apikey/:apikey/inbox', onIncomingSms);