So FaceTime lets you make a link that you can give to someone with a web browser and they can use it to reach you, and it works pretty well. You might just try it.

> I suppose I'll have to talk my parents into installing Signal desktop, but I was kind of hoping for something that gave you the "user is online" status thing like Skype does.

That's probably the biggest limitation: It's a webpage for calling you (the person with the iPhone), not a page for you to call them. If you want them to open a app/page when they are available, I think Messenger is best in terms of features and usability.

If your parents/grandmother aren't already on WhatsApp I don't think you should link their phone number (which might be linked to their banking etc) with a public chat system because there are a _lot_ of online scams targeting the elderly through WhatsApp, Signal, Telegram and linking to the mobile number associated with other (higher value) services. It is very easy to lock-down Messenger so nobody who isn't already a friend can't target them.