Trying to avoid a bit of unnecessary confusion, I would call the SIM
"security code" only as a "PIN code". The phone may have a similar code but
my phone manual names it as "a phone security code".

The phone security code, depending on the phone brand,  can be used as
additional protection against theft, a thief can not use the phone with
another SIM card if the phone security is activated (doesn't help a lot
though if you lose your phone). Another option is to disable phone use
(calls) at any time without a phone security code being given first. This
would help against a thief calling with your phone with your SIM card (if
the phone was on and was kept on so that the SIM PIN code is not needed).