One way is to find a column that will always have data in it if the row is used.

Then (in code), go to the bottom of that column and come up to that last used
row.  Then drop down one row.

I used column A in this sample:

   Dim DestCell As Range
   
   With ActiveSheet
       Set DestCell = .Cells(.Rows.Count, "A").End(xlUp).Offset(1, 0)
   End With
   
   'your code for getting the values
   CallCost = InputBox("Enter the cost of the call")
   
   With DestCell
       .Value = TelephoneNumber
       .Offset(0, 1).Value = CallDate
       .Offset(0, 2).Value = CallTime
       .Offset(0, 3).Value = Duration
       .Offset(0, 4).Value = Description
       .Offset(0, 5).Value = CallType
       .Offset(0, 6).Value = TimeBand
       .Offset(0, 7).Value = CallCost
   End With

You can use that "Set DestCell..." line over and over and over (in a loop).  As
long as column A is always used.

Or you could keep track and just use it once.  Then after you fill out a row,
you drop down one row and start over.
 
   With DestCell
       .Value = TelephoneNumber
       .Offset(0, 1).Value = CallDate
       .Offset(0, 2).Value = CallTime
       .Offset(0, 3).Value = Duration
       .Offset(0, 4).Value = Description
       .Offset(0, 5).Value = CallType
       .Offset(0, 6).Value = TimeBand
       .Offset(0, 7).Value = CallCost
   End With

   Set DestCell = Destcell.offset(1,0)

It kind of depends on how you write your loop.

===========
And just as a sample, you may want to look at how Debra Dalgleish uses a
UserForm to accomplish the same kind of thing:

http://contextures.com/xlUserForm01.html

You may be surprised to see how straightforward it is.