i was on the beach when my mobile phone rang

The last summer, i was on the beach with my friends when my mobile phone ( Mobile phone is commonly said in the UK. It is cell phone when speaking American English) rang. My sister needed my car and she said it was very important. When I got to my house, mi my sister was very nervous because the dog had was lost!
She went to the police station with my car, and I was near to my house, but we didn’t find it.
Three hours after later, we got to my home very sad when we heard a noise into inside the closet. We opened it and my dog was sleeping in my clothes!!!

i was on the beach when my mobile phone rang

Last summer, I was on the beach with my friends when my mobile phone rang. My sister needed my car and she said it was very important. When I got home, my sister was very anxious because (her dog was lost/she'd lost her dog/the dog was missing/lost).
While she went to the police station in my car, I waited/searched near my house but couldn't find it.
Three hours later, we went back home very sad when we heard a noise in the closet. We opened it and my dog was sleeping in (on? - "in" would imply between items of clothing or even that your dog was wearing your clothes!) my clothes!!!

i was on the beach when my mobile phone rang

The Last summer, I was on the beach with my friends when my mobile phone rang. My sister needed my car and she said it was very important. When I got to my house, my sister was very nervous because the dog had was lost!
She went to the police station with my car, and I was stayed near to my house, but we didn’t find it.
Three hours after later, we got to my home feeling very sad when we heard a noise into the closet. We opened it and my dog was sleeping in on my clothes!!!

Very funny story! Your English is excellent, keep it up :)