but lets imagine that somone open a nwe wallet on trust and start using it without even keeping the phrase...is there a way to enter his wallet and make transactions without even having the back up phrase?
If the wallet was opened on a particular device then yes it can still be accessed on that device, but if someone else is trying to use another device to access the wallet, then thats when it will require the private key or seed phrase in order to access the funds in that wallet, there's absolutely no way anyone can enter into a wallet from a new device without having the private keys to that wallet, it's impossible have never heard of such.