CONNECTION PROBLEM OR INVALID MMI CODE
Every time I dial special network commands I always prompted with error: “Connection problem or invalid MMI code”. The number that I was trying to dial is "*199#". It could be due to any recently installed app that which is causing this error in my phone or maybe I am using custom ROM. This problem is very frustrating until I figured out that the work around is very easy.
One way of solving this problem
One way of solving “Connection problem or invalid MMI code” is to save the special number to your contacts and a comma ',' at the end. Try calling the special number again, it should be good to go! Enjoy! 🙂