You can easily buy and send postcards almost everwhere in Warsaw. If you want to send the postcard you should put it into one of the post office red boxes. The name of the post office is Poczta Polska (page is only in Polish).
The stamp for an "economical" postcard or letter (i.e. up to 6 days for Europe) costs 5.20 zł and it does not depend on the destination.
The costs mentioned above apply only for postcards and letters up to 50g - for larger letters or parcels the cost will depend on destination and weight.
Postal stamps may be purchased in Postal Offices ('Urząd Pocztowy' in Polish) and some (few) newsagents. Some of the souvenir shops can sell you stamp together with a postcard.
Public payphones are a thing of the past, but they are still widely avaliable throughout the city. In most cases to use a payphone one has to purchase a calling card from newsagent (kiosk). Majority of payphones accept only calling cards with a chip. When calling regular number from stationary or mobile phone you have to dial the area code (for Warsaw it is 22) and the number. In case of mobile phones the trailing country code may be needed. For numbers in Poland use +48 as country code.
There are four major mobile operators in Poland that provide roaming. All of them offer full range of 2G (GSM 900/1800) and 3G (UMTS) services. They also offer, to various extent, 4G (LTE) roaming services. It is advisable to check with your mobile operator which Polish network is recommended for roaming services. For more information please follow the links to mobile operators' pages:
All four networks mentioned above offer very good prepaid SIM's for voice and Internet.
Note: Please, be advised that voice and data roaming charges in Polish mobile networks are regulated by strict EU directives protecting consumer from excessive payments. Please make sure that you take advantage of these measures, and that your native mobile provider does not overcharge you on top of actual roaming fare.
Conference organisers will make every effort to provide decent WiFi network access at the conference venue. Access to Internet in hotels should also be possible, although some hotels still charge for that.
There are several free hotspots in central Warsaw, mostly located in cafes and large stores. In the main touristic spots, including the vicinity of WIC 2014 venue, the free WiFi access is provided by the city authorities.
It is also very easy to get a reasonable prepaid data SIM card (3G/4G) for your phone or tablet from one of the mobile telephony providers listed above.