For storing your Callcards, it is very important to choose holders or containers that are Acid and PVC free. Both these can cause irreparable damage to your collection so make sure the folders/containers you are using are suitable. If you are looking into purchasing folders/sleeves for your Callcards I would recommend using the UltraPro trading cards pockets and sleeves for your cards.
These items can be picked up online (on ebay) or in most comic book/game shops that sell trading cards. For sleeves the "Yu-Gi-Oh" sized for small gaming card ones work the best for Callcards, I find, however I do know of collectors who use the default card size ones with success.
The old Telecom Eireann and Eircom ring binder collectors albums are also believed to be acid free, so should be perfect for storing your Callcards.
For storing your swap cards, particularly valuable rare ones I would reccomend putting each card in a sleeve (such as the UltraPro Acid free ones) and keeping stored in a closed box away from sunlight or heat. A common occurance for Callcards, particularly among certain manufacturers is sunburn. This is where the card is damaged and goes a darker/orange colour from extreme heat/sun light.
When storing your swap cards you should also try not to have multiple cards touching off each other. This will make the Callcards more likely to receive scratches and wear marks, devaluing them.