You must transfer the phone number when transferring  from GSM to GSM on the web site.
If you only want to transfer the minutes and days from an active GSM phone you will have to call Customer Service.

The inactive used phone has to have a brand new sim card.  There are two types of Tracfone sim cards (C4 or T5), the sim cards in both phones have to be of the same type.  Once a Tracfone is turned on with a sim card in it, that sim card is locked to that phone and can not be used anywhere, ever, except in that phone.  You cannot switch sim cards in Tracfones unless they have never been in a powered up phone.  A used or active sim card in the wrong phone will display a "sim reject" message.  Some phones display the message constantly, some display it momentarily at first power up.  An "unregistered sim" message is not the same as "sim reject".  All sim cards are unregistered until they are activated with a cellular carrier.

You need to have an account to transfer from an existing phone. Set up an account and put the active phone in the account. 

This tutorial is only basic. I do not go in depth with each page. If you have problems you can contact me at the link at the end of the tutorial.