It is simply saying that the contents of phoneGridView.SelectedRow.Cells[4].Text are not recognisable as a valie TimeSpan in string form. We can't fix that: it's your data that is the problem!
Have a look at the cell content, and at the Parse method description / example :
http://msdn.microsoft.com/en-us/library/se73z7b9.aspx[
^] - I don't know what your data looks like, so we can't correct your problem.