I use Set Page Variable to set variable.
Logic
just simple SUBSTRING(pageVars.MobileNumber, 0,11)
and put the formula also on Toast to see the output.
as you can see. on Toast show right but on Input Field are not.
Hmm I tried this out and I don’t think substring will work for this 
But there will soon be a maxLength property for input (with 4.2.X) that will do the same thing as you want to achieve here! 