Here is an excerpt from fop2.cfg, you will have to use the option that is more appropiate for your scenario/asterisk version:
; Call pickup uses the pickupmark variable by default. In multi tenant
; systems this might lead to problems as you might end un picking up
; some other tenant call. In that case you might want to try to
; pickup the call by its context uncomenting the following line:
;
; no_pickupmark=1
; If your asterisk version supports the pickupchan application it is
; much better to use this than the regular pickup application as it will
; be directed towards the channel and not the extension, makeing it
; more precise.
;
; use_pickupchan=1