2 things from personal experience. I used to live in UK and get a lot of flagship phones until 2 years back. Every year, I used to bring and sell 5-6 phones in Islamabad market between me and my brother.
We used to carry these phones with us in luggage and hand carry. The customs never checked or asked us any question.
However, point no 2, I know for a fact that GSM frequencies particularly for North America market are very different to those in use in Pakistan. If you manage to bring a phone from US/Canada, they will most likely only work on 2G/3G at most. Their LTE bands are very different to Pakistan.
The best approach will be to get phone sets from Europe and Middle East, where same bands are used.
HTH.
Tapatalkin' from my Sony Xperia Z3 Dual 4.15.10