"When I see something like 'Can I call you?', it implies to me that there's something really important you need to tell me, that's too important to send over text. I know you don't mean it that way, but it's very scary to me in the absence of more information. Can you please include a reason why you'd like to talk next time you text?"

Once my folks understood that they were putting across the subtext of a death in the family, they started adding why they wanted to call!