You are approaching this in quite an odd way. You already have a reversing function, so why not make line just build the line the normal way around?

def line(bottles, ending):
    return "{0} {1} {2}".format(bottles, 
                                plural("bottle", bottles), 
                                ending)

Which runs like:

>>> line(49, "of beer on the wall")
'49 bottles of beer on the wall'

Then pass the result to reverse:

>>> reverse(line(49, "of beer on the wall"))
'llaw eht no reeb fo selttob 94'

This makes it much easier to test each part of the code separately and see what's going on when you put it all together.

Integer numbers don't have len().

Testing if a number is a palindrome is as simple as testing if the number is equal to its reverse (though if you want maximum efficiency you can just compare characters from both ends of the string until you reach the middle).

To find the reverse of an integer you can either do it the hard way (using mod % and integer division // to find each digit and construct the reverse number):

def reverse(num):
  rev = 0
  while num > 0:
    rev = (10*rev) + num%10
    num //= 10
  return rev

Or the easy way (turning the number into a string, using slice notation to reverse the string and turning it back to an integer):

def reverse(num):
  return int(str(num)[::-1])