The crudest level of phreaking is known as shoulder surfing, which is simply looking over another person's shoulder who is typing in a calling card number at a public pay phone.
The prime locations for shoulder surfing are airports because travelers are more likely to use calling cards rather than spare change to make a call. Given the hectic nature of a typical large airport, few people will notice someone peering over their shoulder while they punch in their calling card number, or listening in as they give it to an operator.
Once you have another person's calling card number, you can charge as many calls as you can to it until the victim receives the next billing statement and notices your mysterious phone calls. As soon as the victim notifies the phone company, they will usually cancel that calling card number, and you'll have to steal a new calling card number. Since it is theft, true phone phreakers look down on calling card number stealing as an activity unworthy of anyone but common thieves and juvenile delinquents.