An inter system handovers in a mobile telecommunication system is performed when a dual mode user equipment (UE) covered by both GSM/GPRS network and UMTS network connects a dedicated channel and sets up a call in a BSS region where the GSM/GPRS provides coverage, and then moves to a UTRAN (UMTS Terrestrial Radio Access Network) where the UMTS provides coverage, wherein the method includes the steps of: if the dual mode UE receives an inter system handover command, i.e. from the BSS to the UTRAN, requesting GSM/GPRS data link layer to suspend a GSM/GPRS data link by a sublayer RR of GSM/GPRS network layer in the UE, requesting a GSM/GPRS physical layer to release a physical channel of the GSM/GPRS, and sending the inter system handover command to the UTRAN for authorizing a sublayer RRC of UMTS network layer in the UE to continue a call; requesting, at the RRC in the UE, a UMTS physical channel to be configured as a UMTS physical channel, and monitoring if the UMTS physical layer succeeds to have the configuration of the UMTS physical channel as requested; if the UMTS physical layer succeeds to have the configuration of the UMTS physical channel, requesting, at the RRC in the UE, a UMTS data link layer to configure a UMTS data link, and conveying information to the UTRAN through a UMTS channel that the handover between systems from the BSS to the UTRAN has been successfully performed; and sending, at the RRC, a GSM/GPRS resource release message to RR/GRR, thereby resetting the GSM/GPRS physical layer and the GSM/GPRS data link layer.