I have a Nexus 7 3G and I often need to send network service commands (e.g., to use top-up codes or to activate/de-activate data-plans), but can't find a way to do it from the tablet. Instead I'm forced to take the SIM card out of the Nexus and put it into a mobile phone temporarily. Is there a way to do this directly from the Nexus 7?
I think USSD Checker is the app that you are looking for.
It allows you to send USSD codes to your network without the use of a dialer. I haven't tried it myself though so comment whether it works or not would help.
came across this, check if it helps. ......
i'm in indonesia, i use nexus 7 lte 2013, first i do is unlocked n root it. then i install ussd dialer from play store... voila.. its worked.. last thing i do is install xposed framework n xposed sms module.. no problemo with pre-paid card... heheheheh