Do you use an online diary/calendar such as Google or iCloud? If so, here's a neat site to try. You Can Book Me (https://youcanbook.me) lets you link your blog, web site or email to your calendar so that people can book appointments straight into your diary automatically.
If you run a small business, for example, you could include a link on your web site that would allow people to request you to call or visit them. They could choose from a list of available times that you have defined, and reserve a slot without you needing to be involved.
You could even use it to schedule a meet-up with a friend.
Accounts on the system start from $7 per month but a basic account is free forever, so you can try out the concept for yourself. Just go to https://app.youcanbook.me/#/register and register.