Translation:The cellphone was broken by my younger brother.
The phone was broken by my brother.
Not many people say 'younger brother'. Not many people say 'cellphone'.
"Little brother" is more common than the formal-sounding "younger brother". If you have a little brother and a big brother it's very common to use the "big" or "little" and not just "brother" on its own. Especially if you're talking to somebody who knows you and your brothers.
Americans say "cellphone" or just "cell". In Australia we say "mobile phone" or "mobile". But these days they're the most usual kind of telephone so lots of people just say "phone". All versions should be accepted.
If we mean something other than cell/mobile we say landline. The distinction in phone numbers (04 for mobile) also helps keep those clear for us. - Melbourne, Australia
"My little brother broke the phone" should've been accepted, but was rejected.