Aides to the local sheriff and district attorney defend the call surveillance. In calls made from jail a message is played warning that the call is subject to recording and monitoring, so they say those on the call know the conversation is not private or privileged. Last year the sheriff implemented a system allowing unrecorded inmate calls to a lawyer’s landline, once the lawyer submits an affidavit listing that landline number.

And, they say, lawyers can always go to the jail to speak to clients in person.

But to criminal-defense lawyers in New Orleans, all of that is a fig leaf: Most of the lawyers who represent inmates are badly overworked public defenders carrying 150 felony cases or so at a time. The notion that they can routinely take an hour or two to go to the jail to see a client — or that they are likely to be at a landline when a client is able to call from the jail phone — is absurd, they say.

Some criminal defense lawyers gave up landlines long ago, too, and only use cellphones.

“I don’t know a lawyer who still has a landline,” said Nandi Campbell, a private criminal defense lawyer in New Orleans. A few years ago, Ms. Campbell approached the prosecutor in one of her cases with a lowball plea offer, though she and her client had talked about their willingness to accept a longer sentence.

“He told me he knew the real number I and my client were discussing,” Ms. Campbell recalled. “That’s how I knew he was listening to my calls. I was startled.”

Jailed clients are already at a disadvantage when it comes to planning their defense. Inmates are less able to help attorneys find witnesses or gather other information. And since they are incarcerated and not working, they face more pressure to plead guilty, and they have less money to pay a private lawyer.

Ken Daley, a spokesman for Mr. Cannizzaro, declined to specify how often prosecutors listen to clients calling their lawyers’ cellphones, but he said: “Any call that is on that monitoring and recording system is basically fair game.”

Mr. Daley said the warning that plays at the beginning of these inmate calls constitutes “a voluntary waiver that vitiates privilege” for anyone on the call.