Your massage appointment is due to start in ～ minutes' time
I am calling to remind you that your appointment is due to start in ~ minutes' time.
「後○分でご予約のマッサージが始まります。」＝ Your massage appointment is due to start in ～ minutes' time
もっと丁寧にいうためにこちらのフレーズを使えます。
I am calling to remind you that your appointment is due to start in ~ minutes' time.
イギリス 
