I would like to share my experiences with trying to get another carrier to unlock my iPhone. I have been very persistent and have tried different tactics.
Vodafone UK:
Vodafone will not unlock your iPhone unless it is a Vodafone device. I have used a UK friend's /family's Vodafone Pay Monthly account details with my non-Vodafone iPhone's IMEI. They simply reply with "Not a UK Vodafone Device". I tried this about 15 times
I also bought a UK Pay As You Go Vodafone SIM in conjunction with a Gevey Ultra SIM, loaded it up with credit and made a few calls. This pairs the device with the SIM card. I then reapplied for the unlock and received the same negative reply.
O2 UK:
I used a friends monthly account details, went online and completed the O2 unlock form. 24hrs later I received a response saying "That my device has not been used this the phone number /account".
Therefore I purchased an O2 microsim from an O2 store, installed it with my Gevey Ultra SIM, loaded the account with £20, made a few calls to pair the device and completed the online unlock form.
£15 was debited from the balance within a few hours and a few days I received a text stating that they are unable to unlock my iPhone as it is not an O2 Device.
Via their LiveChat Service, an Agent manually submitted my request on a priority basis and was told that it will be 100% unlocked within 24 hours. I did this five times with no success.
They submitted the request to Apple but it came back as unsuccessful with the reason "Carrier Mismatch".
Therefore Apple will NOT unlock an iPhone which does not belong to that Carrier. They refunded my account with the £15 charge.
By the way, I spent 3 weeks in the UK with the Gevey Ultra SIM and it did the job nicely.