One circumstance: a lot of men* are in prison for domestic violence. The victims are often close to completely emotionally and mentally broken. The offenders often have equally manipulative friends who are not incarecerated. Allowing such prisoners unfettered phone access to 1) potentially contact victims or 2) contact friends to harass victims, seems like it is worth avoiding. Words can be very powerful and escaping the cycle of abuse is difficult. That was the first instance I thought of. I imagine there are many other where censorship makes sense.

* this is the most common dynamic, violence perpetrated by men.