Can an attorney stop an authorized person from calling him about the case?

No, but it depends on what is being said about the case. Since all of the calls are being taped, and might end up as evidence in the courtroom, the attorney in question might be giving the inmate good advice to remain silent. Why talk about a case when the other side can hear what you're saying. If the offender implicates themselves in the conversation, you'll will wish you never spoke on that phone line.