Hi ​@slats1970 

This is the info I have found with regards to your question:-

Phones sold by iD Mobile are not locked to their network and can be used with other SIM cards. However, there are exceptions, particularly with older iPhones, which may have been locked before 2018.
In the UK, mobile phone companies were banned from selling locked handsets (phones locked to a specific network) starting in December 2021. This means that any new phone purchased from a mobile network provider after that date is sold SIM unlocked.

As you are looking at an  iPhone 16 pro max, this should be OK as it is a later model.

Although please be aware, some 2nd hand phones sold on selling pages e.g. EBay, may be sold as ‘unlocked’ but that is only with regards to being able to swap SIMs between different networks BUT it may still be ‘locked’ with regards to software updates (where the phone is required to be connected to the network it was sold by to get any new software updates rather than being ‘unlocked’ and getting the updates directly from the phones manufacturer)

Phones sold by iD Mobile in the UK are not locked to their network for software updates. They are generally unlocked, meaning you can use them with any compatible SIM card and receive software updates from the phone's manufacturer. 

Hope this helps x

Hi ​@slats1970 

Yes, all new phones sold by iD Mobile are completely unlocked.

Tom