FWIW, this article states that MVNOs in Poland have a grand total of 130,000 users. If so, I can't imagine that many are doing particularly well. If, say, Carrefour has one-third of all the MVNO business, that's only around a piddly 40,000 users; I get the impression they were expecting several times that. Which might explain all the extra advertising they've been doing.
Total Telecom - Poland's mobile penetration rises to 116.8% in Q3