Well it depends on how your parents are and if they were the one buying the phone for you. Obviously if you're financially independent, it doesn't really matter, isn't it? Just tell your parents you lost your phone, contact the phone company to see if you could retrieve the number at least.
If your parents buy you the phone, and says... they are quite strict, but not unreasonable, and if you're sure that the phone is gone, and already did everything you could to find it, eg look for it at the theatre, tried to call it etc... Honestly just explain to them that you somehow lost it and apologize. The sooner the better, wouldn't do if there are some kind of emergency or they need to contact you and can't, they'd be worry for sure.
If for some miraculous reasons your parents are super fun and/or have no qualm about money, then tell them that you discarded your old phone as a way to terminate your bad relationship with the year 2019, and require a new phone for a new decade
)
Edit: I forgot to say if your parents are super strict and you know they'd be upset, then brace yourself for some beating and tell them anyway :v