Be careful there. The mere existence of a DTA is not enough to say that the witholding tax can be reclaimed. It depends which country gets assigned the right of taxation as per the DTA… If it happens to be CH then the withholding tax won’t be reclaimable (but that is usually desirable, given the low tax rate of WHT in CH).
Check out this map to know where the WHT is reclaimable, or not.
No, it cannot be claimed back!