can a blocked phone be unblocked (phone was lost/stollen)

My phone was lost and stollen as it wasnt returned to me... someone did find it and decided to ignore my hundreds of calls to give the opportunity to give it back.. i know someone had it as my network said they accessed the internet but didnt make any calls.. so had the phone and sim blocked.

Someone told me this menas that they cant get the phone unblocked as it was blocked by the network. I thought they would be able to take it to get unblocked from the umpteen places that do it these days, but after a wee google search seems it may be true that they cant....

anyone know anything about it? thanks x