OTE offices exist all over Greece. The larger the town the more offices. Almost every island has one.
Left is the OTE office on Athinas Street near Monastiraki Square.
You can call home from here. Athinas St. connects Athens' Omonia Sq. with Monastiraki Sq. and is major street with many inexpensive shopping resources for travellers.
At the OTE office, you enter a private booth, sometimes after waiting in line, and can place calls to anywhere in the world.
You will be charged according to the little meter monitoring your usage of monades or units.
This will be less expensive than from your hotel.