I work for a major telecom in the prepaid card division. This is a declining market as it relates to Long Distance. The calling cards have been replaced by "free" long distance on cell phones for those who can afford them. Margins have shriveled to nothing. The traditional market for prepaid LD is low income users, travelers, and college kids. Low income users buy cards as cheaply as possible and may even visit bodegas. No profit there. Travelers and College kids are switching to using their cell phones, so lack of opportunity. Additionally the wholesale clubs (Sams, Costco, BJS) have such low rates compared to other retail rates you will not likely have a competitive product.
Where you might have some luck is in the prepaid wireless business. Still it is a hard sales job, when one can walk into just about any pharmacy, electronics store, or even convienence store and pick up a refill.
Recharge is also a limiter for you. Once someone has a card they can add more minutes using a credit card, rather than buying a new card from you.
There are other prepaid products emerging but most are retail loyalty based.
The calling card industry is not expected to survive more than a few more years. I would not enter into it as a full time job at this point. Unless you live in Alaska or Hawaii, there are opportunities due to pricing regulation and lack of stores carrying prepaid cards.
Things may be different in Europe and Asia markets.