I am unable to disable the dial assist feature on my Iphone 4 (IOS 6.1.3), I have checked Settings > Phone and do not get a dial assit option .. please help I have looked everywhere and even reinstalled my IOS software but no help

If you scroll down to the bottom of the phone settings section, you do not see the Dial Assist switch? What carrier do you use? Did you purchase the phone from the carrier or Apple?

I would have to guess, is this a supported provider for the iPhone? If so, the carrier file is preventing you from making this change. You will need to check with the cellular provider about that. The carrier profile that is on the phone can add/delete settings options on the phone based on what they allow/disallow.

Coincidentally I have had the same problem and I am also on Reliance network. I learnt the lesson in an expensive way (Read at Rs.17/min as this is the ISD rate of Reliance for calling to USA). I did mnp (mobile number portability, you oldie!) from Airtel to Reliance. So I had the inactive nano sim of Airtel. I took out the Reliance sim and inserted the Airtel sim into my iPhone 5s. By the time it was serching for the network, I quickly navigated to settings>phone and turned that ****** dial assit off. That's it. Now when I inserted the Reliance sim and dialled the toll free numbers it was working perfectly, means free of cost. But I still wonder how come Reliance does not send the post call charge notification after the ISD calls? I had to call 198 to know about the balance deduction.