No. This is a common assumption that people have when they have only used one network ever on the phone, without testing a different networks SIM card.
A large proportion of phones are purchased on-contract with a network, often with the up-front price of the phone itself being vastly reduced or "free". Usually, this is because the network have locked the phone to their network, allowing them to subsidise the upfront costs of you purchasing it over the lifetime of the phone.
However, not all phones purchased on a contract have been locked successfully. There have been numerous instances when someone has received a phone via this route and the phone was already unlocked. It is always worth checking before placing an order with us that your phone is locked.
A lot of people also buy phones second-hand, from sites such as eBay or purchase them from a store of a PAYG basis. Not all of these handsets are locked either and it shouldn’t be assumed that they are.
Handsets that “should” be locked, like the ones usually bought through the methods mentioned above, can be missed by the manufacturer. This means that even though the handset was meant to be locked at the request of the networks, sometimes they aren’t for whatever reason.
The final method of phone purchasing is “SIM free”. For instance, generally any iPhone purchased directly from Apple is SIM free. Handsets can also be bought in this state directly from mobile phone stores. “SIM free” simply means that the phone hasn’t been subsidised by a network and isn’t locked to one. Therefore the buyer will have to pay the full retail cost of the phone. People who purchase via this route often already have a network they have a contract with and want to continue using. Alternatively, they could have a SIM only contract or PAYG SIM card that is preferred.