- to - one ▁walk ie ▁talk ie ▁but ▁without ▁the ▁normal ▁range ▁limitation ▁since ▁the ▁call ▁still ▁uses ▁the ▁network . ▁T ET RA ▁terminal s ▁can ▁act ▁as ▁mobile ▁ph ones ▁( cell ▁ph ones ), ▁with ▁a ▁full - du plex ▁direct ▁connection ▁to ▁other ▁T ET RA ▁Users ▁or ▁the ▁P ST N . ▁Emer gency ▁buttons , ▁provided ▁on ▁the ▁terminal s , ▁enable ▁the ▁users ▁to ▁transmit ▁emer gency ▁signals , ▁to ▁the ▁dispatch er , ▁overrid ing ▁any ▁other ▁activity ▁taking ▁place ▁at ▁the ▁same ▁time . ▁ ▁Adv antages ▁ ▁The ▁main ▁advantages ▁of ▁T ET RA ▁over ▁other ▁techn ologies ▁( such ▁as ▁G SM ) ▁are : ▁▁ ▁The ▁much ▁lower ▁frequency ▁used ▁gives ▁longer ▁range , ▁which ▁in ▁turn ▁perm its ▁very ▁high ▁levels ▁of ▁ge ographic ▁coverage ▁with ▁a ▁smaller ▁number ▁of ▁transmit ters , ▁thus ▁cutting ▁infrastr ucture ▁costs . ▁ ▁During ▁a ▁voice ▁call , ▁the ▁communic ations ▁are ▁not ▁interrupted ▁when ▁moving ▁to ▁another ▁network ▁site . ▁This ▁is ▁a ▁unique ▁feature , ▁which ▁d PM R ▁networks ▁typically ▁provide , ▁that ▁allows ▁a ▁number ▁of ▁fall - back ▁modes ▁such ▁as ▁the ▁ability ▁for ▁a ▁base ▁station ▁to ▁process ▁local ▁calls . ▁So ▁called ▁' mission ▁critical ' ▁networks ▁can ▁be ▁built ▁with ▁T ET RA ▁where ▁all ▁aspects ▁are ▁fail - safe / multiple - redu nd ant . ▁ ▁In ▁the ▁absence ▁of ▁a ▁network , ▁mob iles / port ables ▁can ▁use ▁' direct