Your friend is in their custody.
Unlike America, prisoners DO NOT have the right to a telephone call or to visitors in a watch house.
So no one can see their friend/family who’s in the custody of the police.
Also, the police probably won’t tell you why they have your friend in custody, because it’s confidential. You wouldn’t want the police telling all your neighbours your personal information, so the police won’t tell you your friends personal information. And they’re not going to go ask the friend if they can tell you about it, because, well, they don’t care and have a million other things to do with their time.