Sim card only
Can I use my iPhone with a Sim card only, without activating an Apple ID - or skip the iPhone set up?
iPhone 6s Plus
Question marked as
If the iPhone is carrier unlock you can use your own SIM card.
You need also to sign in with your Apple ID
See -> Set up your iPhone, iPad or iPod touch
Set up your iPhone, iPad or iPod touch – Apple Support (UK)
See also this Apple article: How to unlock your iPhone for use with a different carrier
How to unlock your iPhone for use with a different carrier - Apple Support
Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.
You need to contact your Carrier.