Is it permissible for the seller to retrieve and use the remaining amount (cents) in international calling cards after the buyer has used them?

Selling telecom cards is permissible, as you mentioned. If the buyer leaves a small balance on the card and definitively expresses their desire to abandon it and not use it, then it is permissible for you to benefit from it. This is because it is money that its owner has relinquished. Scholars have stated that whoever abandons their money out of disinterest, there is no harm upon whoever finds it to take possession of it. Ibn Qudamah stated in "Al-Mughni": "And whatever people discard out of disinterest... whoever comes across any of this first is more entitled to it, and does not require the permission of the Imam or anyone else."