You should only receive the actual value of the stamps and phone calls and you should return the change to the customer unless they willingly waive their right to receive it. The proof for this is that the Prophet (peace be upon him) said:
The property of a Muslim is unlawful (to be taken), unless (he gives it) willingly.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.