Prolonged services are add on codes, used to indicate that the physician or Non-Physician Practitioner spent 30 minutes more than the typical time for that code with the patient.

See the Codapedia articles on prolonged services in the office (face-to-face) and prolonged services in the hospital (face-to-face). That article providers detail on using codes 99354-99358.  This article is an overview.   See the article about non-face-to-face prolonged services, as well, for more detail about those codes.

There are two sets of CPT® codes which indicate prolonged services.  All of these codes are add-on codes.  They have  + signal in front them in the book, which indicates that they may never be used alone.  They must always be used with another code.  In the case of prolonged services, there are specific, limited codes on which they can be added.  By CPT® rules, prolonged services may not be added to any code in the CPT® book.

There are two sets of codes: those with face-to-face contact and those without face-to-face contact.  The non-face-to-face codes (99358-99359) are generally not covered by insurance companies and are never covered by Medicare.  Medicare's policy on these is so short, it is reproduced in its entirety below:

30.6.15.2 - Prolonged Services Without Direct Face-to-Face Patient
Contact Service (Codes 99358 - 99359)
(Rev. 1490, Issued: 04-11-08, Effective: 07-01-08, Implementation: 07-07-08)
 
Contractors may not pay prolonged services codes 99358 and 99359, which do not
require any direct patient face-to-face contact (e.g., telephone calls).  Payment for these
services is included in the payment for direct face-to-face services that physicians bill. 
The physician cannot bill the patient for these services since they are Medicare covered
services and payment is included in the payment for other billable services.
 

Although there are RVUs assigned to these codes, they have a status indicator of B, bundled.  Medicare will never pay for them, because they believe they already have paid you, as part of another E/M service. There are  CPT® and HCPCS codes for Care Plan Oversight and Home Health Agency Certification and recertification.  See the articles about these codes in Codapedia.

The prolonged services codes with face-to-face service are further divided into office/outpatient/home services (99354-99355) and inpatient services (99356-99357).