Most Prague (and Czech) telephone numbers, both landline and mobile (cell), have nine digits. There are no city or area codes, so to call any Czech number, simply dial the unique nine-digit number.
The Czech Republic uses the GSM 900/1800 system, the same system in use around Europe, as well as in Australia and New Zealand. It's not compatible with most mobile phones in North America or Japan (though many mobiles have multiband GSM 1900/900 phones that will work in the Czech Republic). If you have a GSM phone, check with your service provider about using it in the Czech Republic, and beware of calls being routed internationally (very expensive for a 'local' call).
Local prepaid cards for payphones in the Czech Republic include Smartcall (www.smartcall.cz) and Karta X Plus – you can buy them from hotels, newspaper kiosks and tourist information offices for 300Kč to 1000Kč. To use one, follow the instructions on the card – dial the access number, then the PIN code beneath the scratch-away panel, then the number you want to call (including any international code). Rates from Prague to the UK, US and Australia with Smartcall are around 6.6Kč to 10Kč a minute; the more expensive the card, the better the rate.