The first 1-3 digits (after +420) of the telephone number indicates location or network. For mobile phones, since there is number portability, the mobile phone code only indicates the original operator. For example, when a person calls a number starting with 73 (T-mobile) but had been ported to another operator, a short voice message in Czech and English is played stating "you are calling out of a T-mobile network" before the ringing tone.