error: invalid conversion from 'char' to 'char*' [-fpermissive]
You're calling your function with a single char, specified by phoneNumber[i], but your function is expecting an array.

If you change your call to:

LcdString(phonenumber)


then that will pass the entire array.

Are you trying to say that you're storing multiple phone numbers?  i.e. a two dimensional array?  Check out the String reference for more details: https://www.arduino.cc/en/Reference/String