Concrete
I'm thinking that concrete is the actual numbers in their telephone number & the capitalized words in their tweets that contain the letters in the word concrete are how the numbers should be arranged for the telephone number. For America you need a 3 number area code & then 7 number phone number. So I'm thinking the numbers that make up the word "concrete" makes up their telephone # & like I said the words that are capitalized in order that contain the letters that make up "concrete" (some would be repeated to get the full 10 digit number) would make up their number?
Just spitballing. They have to have some way to pass their burner phone number around without others noticing.