SIM Toolkit (App) Missing... Restore??

[AC Question]

Hi. Late yesterday my mobile went on the fritz to say the least ~ my internal phone storage went seriously low and a lot of the android systems started to crash. I was able to uninstall large apps to clear some storage and I didn't touch the SIM Toolkit app but for some reason its gone! How does one get ones SIM Toolkit back?? I am guessing it's something that a phone should have?!
Do i need to do a backup of my phone apps etc. then do a Factory Reset on my phone to get it back? ~ then import the backup of my apps etc.?

Your advice and guidance would be very much appreciated.

Thanks.
Jamie