In uk.railway Stephen Sprunk twisted the electrons to say:
Even between two carriers using the
same protocol, carriers generally "lock" the phones they sell so they
can't be used on another's network; this is the price of getting "free"
or heavily-discounted phones when signing a service contract--but it
also means millions of phones (and their toxic batteries) go into
landfills every year.

Depends on the operator. Vodafone & O2 will, I'm told, unlock brand new
contract phones if requested. They take the view that you're still going
to be paying the monthly contract amount for the duration of the contract
but if you want to pay another network's contract as well then that's up
to you! (One could argue they benefit in that they get the contract
money but don't have the costs of providing the service!)

Alternatively there any number of companies (see http://mobilevaluer.com/)
which will buy old phones and recycle them ...
--
These opinions might not even be mine ...
Let alone connected with my employer ...