The Vodafone network does not generate random TMSI numbers, which allows you to copy them and thereby listen in to other ongoing conversations. The network won’t throw off duplicates. If you have an IMSI catcher you can exploit this. It does, however, put the phone into conference call mode, which shows up on the screen. Considering your screen is mostly next to your ear, most people won’t notice.

Source: Kwetsbaarheid in Vodafone-netwerk maakt gesprekken kopieerbaar | PCM