Why not focus on the fact that you have two parents who both are in such good shape that they get out and about? Rather than focusing on one phone call, how about thinking, "It's terrific they aren't sitting home taking a dozen meds and being depressed at the loss of their mobility"--? Too many of us have parents who can't even get out their own doors due to physical health, or who are suffering the depression that too often comes with old age and makes older people avoid social situations. I hope you can stop to give thanks that your parents are in such good physical and mental shape, and focus on that rather than on one phone call. I'd be delighted to think that my mom was so busy being social and active and out of the house that she couldn't stop to call me just yet. I wish that were the case.
If you're genuinely worried that something has happened to them (and not just sore that they haven't called you), call the local brother and tell him that. He can go out and look for them. You can't do anything, from a distance.