Sometimes you get a detention memo sometimes you don't. If the tracking status says "record item in customs" it means the package has been detained and its not just mobile phones that get detained. Items that are underinvoiced, items that are not allowed as per law, things the need import approval from relevant authorities can get detained by customs. Call Pakistan Post helpline and ask them about the status of your parcel. They also have contact numbers for customs office.
If it is not detained, usually it will arrive within a week after processing by customs. Rarely it can take longer. One of my items kept sitting in customs for more than a month before arriving.