Factory unlocked vs Forced unlocked

downuno

I've always wondered about the difference between the two. Is unlocking a phone that was previously locked to a network exactly the same as a factory unlocked phone? Is a previously-locked phone, after being unlocked, more prone to issues concerning:
-constant connectivity to network
-accepting international sim cards
-voice/texts is fine but data might not work
-etc. any other problem, mention it if it does make a difference.
(or not?)

Second question (sort of related):
Sometimes I see some unlocked phones for sale that say they are unlocked to most networks but won't work with SIM cards from certain networks (e.g. 3 UK). Why is there this separation of certain networks?

Browntoa

question 1

code unlocks remove the network lock , there are no issues with using the phone although you need to make sure you have the new networks settings sent to the phone or entered. Some phones are unlocked by reloading software to the phone which effectively makes it new out of the box unlocked phone

question 2

unless the phone supports 3G network frequencies it will not accept a 3G sim card , ie a Nokia 2600 would not , a 6500 would