iPhone 6(S)/Plus SIM unlocking

[CooperBox]

Hope someone can assist here as my knowledge of iPhones is limited.
I've just been given an iPhone 6S from a family member in Uk who has recently upgraded to a newer model. I'm based in France.
I've just inserted my Sim card which worked ok with any other non-Apple phones, and another iPhone, however it's not being recognized on this 6S, which reports the message, 'No Sim card installed. Insert a valid Sim with no Pin lock to activate phone.' I assume therefore that this 6S is locked to it's UK service provider.
Is it advised that I take it to be unlocked to one of the many small phone boutiques downtown, or contact the original owner to request having the phone unlocked by her original service provider?

 

[MarkX]

I believe you will have to return the phone to the original owner for them to have their service provider unlock the phone.

If I recall I think the process requires the phone in question to be connected to the original service providers network.