It is up to the network to decide what ID type it wants to use to page a mobile, and this is dependent on a number of factors. Almost always it uses the TMSI, sometimes IMSI, and _very_ rarely an IMEI. It is certainly possible that the network can also request identity info (IMSI, IMEI) from the mobile on the dedicated channel, as well as a whole host of other message requests while on the dedicated channel.