Do confirm your APN settings are set as such below :

U Mobile Internet APN
Name : U Mobile Internet
APN : my3g
Proxy, Port, Username, Password, Server : NIL
MMSC : http://10.30.3.11/servlets/mms
MMS Proxy : 10.30.5.11
MMS Port : 8080
MCC : 502
MNC : 18
Authentication Type : none
APN Type : Not Set
APN Protocol : IPv4
APN Roaming Protocol : IPv4
Bearer : Unspecified

The *118# and other similar codes are all called USIM codes. The Nexus 7 does not have phone function, thus not able to support USIM codes.

You have to take out the sim and put into other phone to do all the USIM functions (topup, check balance, subscribe UMI Internet).

or u can use this method...subscribe via sms..

http://www.u.com.my/prepaid/unlimited-mobile-internet-plan

u can also topup using sms too

http://www.u.com.my/prepaid-packages/top-up-locations

VPN or APN? 

At mobile network setting go to access point name and choose umobile 3g.

salam fellow nexus 7 brothers! just got mine today!

Anyone using umobile on nexus 7 3g?
A weird problem appears after i put an umobile sim card on my nexus. After restarting the nexus 7 keeps staying in silent mode. Even though i tried to turn off silent mode by holding the power button and deactivate silent mode. i can't increase the volume by using the volume rocker. All the media player apps including youtube will not play the media content, they js got stuck. After i changed back to my old maxis sim card, the problem will js gone. Any idea how this happens?

TL;DR
Switched to Umobile from maxis, weird problem arised after inserted the umobile sim card, nexus 7c can't exit silent mode, back to normal after i inserted maxis sim card

Or.. you could trigger it manually

I have posted the guide here yesterday. Look for it.