iPhone 11 - locked to network

[9xxNick]

This isn't something I've ever had to worry about before, so would appreciate some straightforward advice on the best option(s).

My wife has bought an iPhone 11 from someone she knows for my daughter to use. However on inserting the SIM (Vodafone) the phone shows a message that I assume means the phone is locked to a network other than Vodafone.

What are the best options for unlocking this phone?

I may be able to find out what network it's locked to from the previous owner but if that's not necessary then all the better.