POP-UP Message in my cell phone??

got a message in my cell, its not a text, coz the whole message just popped up automatically on my screen... and when i press endcall, it disappeared and not in any text folder. it was a personal message from someone i blocked (im using samsung gt-e1080/f, basic cell fone, and they found a way to bypass the text blocking and send this pop-up message.

how did they do it? is this some kind of SOS push message or something?

(my phone is actually samsung gt-e1080/f, its not listed, my service provider is also not listed)

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any help?