Is 2 Factor Authentication by SMS secure?

Episode 1555 (55:26)

Carmine from Chicago, IL
Smartphone and computer

Carmine has 2 factor authentication on most of his systems, but some use SMS, and he thinks that's not very secure. Leo says that there will always be a trade-off between security and convenience. But SMS is far easier to crack than independent authentication through an authenticator. Leo says to contact the cellphone company and have them put their additional layer of authentication on her phone.