We're calling you because it's less than 15 minutes before your appointment time.
If you're late for your appointment, your treatment time will be shorter.
この場合、次のような言い方ができますよ。
ーWe're calling you because it's less than 15 minutes before your appointment time.
「お客様の予約時間前15分を切ったのでお電話しています」
ーIf you're late for your appointment, your treatment time will be shorter.
「もし予約に遅れますと、施術時間が短くなります」
to be late for one's appointment「予約に遅れる」
treatment time「施術時間」
ご参考まで！
Hi, we're calling to let you know that your appointment is in 15 minutes.
ご質問ありがとうございます。
・「Hi, we're calling to let you know that your appointment is in 15 minutes.」
（意味) 予約時間１５分前です。
<例文>Hi, we're calling to let you know that your appointment is in 15 minutes. If you're late your session will be shorter.
<訳>予約時間１５分前です。遅れると施術時間が短くなります。
参考になれば幸いです。
カナダ 
アメリカ合衆国 
