Finding people that "don't call their (grand)parents enough"

Other comments already mentioned the issue with the problem not being critical enough. I would like to dive into this a bit more, with a twist.

In my book, I recommend looking for a validated niche audience before building or even "solving a problem." You question of "where to find those people" indicates that this one is a particular hard audience to pin down. The audience you're looking for is "people who would rather spend money on a tool that reminds them to call their family than mustering the attention to do it themselves"—this sounds like a super hard sell. It might even be a contradiction in itself. "Life gets in the way" is not divine intervention, is a prioritization problem.

But not all is lost. I think you're actually looking at an interesting audience exploration opportunity here. Instead of wanting to build a thing that reminds people to "get in touch", I would flip this whole idea around and look for people who have actual problems "staying in touch" with their family. I know this might sound similar, but I want to point out the nuance. People who already were in touch with loved ones but for some reason get cut off are much more likely to have "getting in touch" as a critical problem. They might already have spent money on phone calls on in-place visits before, so paying for something that facilitates getting in touch is a much stringer "yes" for them. So who are those people? Families that were torn apart by the COVID-19 lockdowns? People who had to move to a different city for their job? People who got caught up in the justice system? Older people who had to move to retirement homes far away?

The twist is that these people all share the same problem, but they will experience it differently and they will need specific solutions that fit their circumstances. By diving into any niche audience like this and listening to their actual wants and needs, you will find a more critical problem, and knowing what they expect, you can build a solution that fits their needs.

Please understand that this solution might by no means contain a reminder-based approach. Maybe you will need to build some digital infrastructure, a marketplace, or some sort of network platform. That's the thing about going audience-first: you only get to have your idea AFTER you've researched the niche, AFTER you have collected information on their problems and how they solve them right now.

I call this the Audience->CriticalProblem->Solution->Product approach. Every step along the way, you get to validate. If the audience isn't willing to pay for anything, what good would it do to build something that then would go unsold. And even with an audience that would pay for their problems to be solved, what good would it do if the solution wasn't feasibly due to government restrictions (thinking about incarcerated people here, for example). Once you have a well-defined and existing audience, once you really know that there are many thousands of people out there who share a problem, THEN you can find out how to help them.

In closing, I want to express that I really like why you want to find those people. I know that human connection is the core of our lives, and you wanting to help with that brings a smile to my face. But trying to sell a product that doesn't solve a validated problem to an audience that might or might not exist is very risky. Finding a validated audience you want to empower, detecting their most critical problem, solving it without interrupting existing workflows and finally creating a product that works in the medium they expect to use it — this will increase your chances significantly.

This sounds like a bad problem to focus on, you can't even know of it real. Cause it's social niceties, yes I should be a better person, yes I will do the right thing but when you give me 2 options even with helping the right one be easier, there is no indication you'd change the choice of actions.
By the same point, you won't get these people search how to solve their problem cause it's a problem of choice...
The thing the works for these niches is pain based ads, but I don't think it's going to be easy to make it viable...

Is this a critical problem you are solving?

Do people spend money on getting into contact with their friends/family?

What more can you do than Facebook and WhatsApp?

My point: this is definitely a problem for people. But this is not something people will pay for to get it fixed.

My 2 cents.