@SlackAPI Can you confirm "tel:" urls are _not_ supported in buttons?
cc @roach
https://api.slack.com/tools/block-kit-builder?blocks=%5B%0A%09%7B%0A%09%09%22type%22%3A%20%22section%22%2C%0A%09%09%22text%22%3A%20%7B%0A%09%09%09%22type%22%3A%20%22mrkdwn%22%2C%0A%09%09%09%22text%22%3A%20%22You%20can%20add%20a%20button%20alongside%20text%20in%20your%20message.%20%22%0A%09%09%7D%2C%0A%09%09%22accessory%22%3A%20%7B%0A%09%09%09%22type%22%3A%20%22button%22%2C%0A%09%09%09%22text%22%3A%20%7B%0A%09%09%09%09%22type%22%3A%20%22plain_text%22%2C%0A%09%09%09%09%22text%22%3A%20%22Button%22%2C%0A%09%09%09%09%22emoji%22%3A%20true%0A%09%09%09%7D%2C%0A%09%09%09%22url%22%3A%20%22tel%3A%2B33176340034%22%0A%09%09%7D%0A%09%7D%0A%5D …pic.twitter.com/2CQm3KKesv
