use XML::Simple qw/:strict XMLin/;

dd XMLin($XML1, KeyAttr=>{}, ForceArray=>[]);
dd XMLin($XML2, KeyAttr=>{}, ForceArray=>[]);
dd XMLin($XML1, KeyAttr=>{}, ForceArray=>['callTo']);

__END__

{ phone => { callTo => "X" } }
{ phone => { callTo => ["Y", "Z"] } }
{ phone => { callTo => ["X"] } }