If you start timer in the following way it might cause problem but if you don't do it in this way it should be fine. Also I would use timer start named_timer 600 timer_1 if I can.

#----------------------------------
# State Machine
#----------------------------------
set fsm(any_state,ev_disconnected) "act_Cleanup same_state"
set fsm(OUTCALL_WAIT,ev_named_timer) "act_Setup same_state"
set fsm(PLACECALL,ev_setup_done) "act_CallSetupDone CALLACTIVE"
....
set fsm(CALLDISCONNECT,ev_leg_timer) "act_Cleanup same_state"

fsm define fsm OUTCALL_WAIT

start_timer <--- start timer procedure