Aside from public phones Poros has a telephone center operated by OTE, the Telecommunications Utility Company of Greece. Local phones are relatively inexpensive. The public phone booths operate with a card (like a credit card) that you can buy from your local general store. They come in many denominations (50, 100, 500, 1000 or 10,000 drachmas).
The area code for Poros is 02 and for the prefecture of Athens is 01. In order to dial from Greece abroad you need to dial 00 followed by your country's code (US is 1).
Fax machines can be found at any hotel, travel agencies and tourist shops. A page takes about a minute to send and costs approximately 1,500 drachmas (about 5 US dollars). Access to e-mail in Poros is not widespread and last time I visited (summer 1996) I did not find a business providing temporary e-mail services although this might have changed since then.