Just use [`Show`](http://reference.wolfram.com/language/ref/Show) instead of [`Overlay`](http://reference.wolfram.com/language/ref/Overlay):

	Show[
		bg,
		ListPlot[
			MapThread[
				Callout[#1,#2,"CalloutStyle"->Directive[Thick,Red]]&,
				plain
			],
			PlotRange->{{-180,180},{-90,90}},
			Axes->None,
			PlotRangePadding->Scaled[0]
		]
	]

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/dnhDv.png