CNAM means Caller Name. Caller Name and Number is a very common premium feature in america and is usually bundled with other premium telephone services like call waiting and call forwarding. Whenever a phone call is being received by a CNAM caller ID subscriber, both the caller name and telephone number are being displayed. It is a service the caller id subscriber needs to pay for.
The caller name being displayed doesn't come from the phone's memory but is passed to the caller id subscriber by the local telephone company. Every time they receive a phone call for a callerid subscriber, they lookup the caller name in a central CNAM database so it can be displayed on the receiving party's telephone display.
Caller ID is available almost everywhere in the usa but there are some states that do not offer complete coverage. Be sure to check with your local telephone company or examine your telephone bill to see if caller id is available.
Caller ID spoofing is the practice of causing the phone network to show a phone number on the recipient's caller ID display that's not that of the real originating caller. Caller ID spoofing is commonly used to describe circumstances in which the motivation is considered malicious.Caller ID spoofing will make a phone call appear to it came from whichever telephone number the caller wishes.
Consequently one should not trust their caller id to identify an important party such as your bank, really don't give out any information over the telephone that can be misused except if you're 100% convinced who you are talking to. Just as with phishing e-mail trying to get your online banking account details.