I purchased some Windows 10 Phones for business use where they are to run a custom UWP business app. Being about $100 they are quite economic but they were locked to local Telco, Optus (Australia). The Telco would give you the unlock key for $90 but I found an online site that could do it for about $20.
The key take home is that the instructions for implementing it as per the Telco and as per the source of the key I used, did not work for me but there is a simple method that worked for me that others had used with AT&T:
The Unlock service I used was http://www.cellfservices.com/store/unlock-nokia-lumia-640.html