nini, for more safety, don't let them over take.
Are you under HR or admin? can come out a memo to announce new rules.
Ask employee to give call to HR. mentioned not allow sms or you accept sms, depend on your company. If apply medical leave should attach medical certificate, if not, treat as absent.
I'm not sure how was your company rules, is it after continuous 12 months service or upon join or confirmed.
If after 12 months, it will not over take.
If upon joined or confirmed, you can mentioned 1 month can take how many days. AL not like MC.