When at user equipment UE transitions between radio access technology cells, at the transition a timer for controlling when the UE is to send a periodic location registration is reset. This avoids the UE being implicitly detached when it is unable/blocked in the packet-switched PS domain from performing a RAU but would be able to perform a LU, particularly when the UE is in PS/CS modes 1 or 2. Five different embodiments are detailed: the LU timer T3212 is reset with a value of time remaining in a tracking area update TAU timer T3412; T3212 is reset with an initial value for T3212; the RAU timer T3312 is reset with a value of time remaining in the TAU timer T3412; and the T3212 or T3312 reset is conditional on a value of time remaining in a PS backoff timer T3346 exceeding the value of time remaining in the TAU T3412.