How can I make a button link to an email?

You should use the mailto: link feature. Basically, you should put this to the button's link field:

mailto:test@example.com

This code will open the installed email application and fill the "to" field with the given email address. In the sample's case, this address is test@example.com

Set subject

You should put a ?Subject= after the email address like this:
mailto:someone@example.com?Subject=Hello%20again

This will make the email's subject "Hello Again"

Can I do the same with phone calls?

Sure. Use the tel: prefix instead.

Example:
tel:+6494461709