I know that this device was ordered in the UK, but I don't know the carrier. Here's the imei check result:
Model: IPHONE 5S GRAY 16GB GSM
IMEI Number: 352049064397434
Find My iPhone: OFF
Estimated Purchase Date: 21/03/16
Coverage End Date: 21/03/17
Product Sold by: THE CARPHONE WAREHOUSE LIMITED
Initial Carrier: UK Carphone Flex activation policy
Purchased In: GB - United Kingdom
Simlock: Locked
Please reply,
Brendan