I do not believe that Play has its own network
Trust me on this (I know a fair bit about P4, and about Polkomtel): P4 have their own network. They do have a domestic roaming agreement signed with Polkomtel, but they are not a VMNO (virtual mobile network operator).
Right now from the web I'm using ''pinger.com/textfree/'' it works 100% sending sms to any Cellphone company in Poland, Once you get androidphone you can send sms international getting app from android market
Thanks for the tip, I'll give that a try (already have an Android phone).