I think not much
the biggest problem all telcos have is filtering international calls ... and some "other porovider" calls which only pass on billing details ... so transylvania 555 5555 passes call to england ... says I want 555 7777 ... just send the bill to transilvania telecom ... who then then bulk bill vlad media for their useage ... no names ... no packdril.
in country can be similar ... depending on what the agreement is.
while the police can go so far ... the cost is huge ... so unless they threaten to nuke you ...