In the current version the Call client <phone number> button in the order detail popup is a plain link with no destination set (its link target is empty), so tapping or clicking it does nothing — it does not open your phone's dialer or any calling app. This appears the same way for both pickup and delivery orders. Read the phone number shown on the button and dial it manually (or look the customer up under Customer Orders / Users). Treat this as a known limitation rather than something you can configure around.