It still is a little. Please correct my understanding if wrong:
An external caller dials a DID that is on your FreePBX. Based on the number called or an IVR option selected by the caller, FreePBX routes the call to the NEC and it rings a local extension. A person answers the NEC extension and now wants to transfer the caller to an external number.
Assuming the above, there are two options:
One is you do nothing special and use the normal call transfer facility of the NEC, e.g. Transfer button on the phone. This will make a new outgoing call and the NEC will bridge the calls together. Depending on the new number dialed and how the NEC is configured, it may route the outgoing call via your FreePBX.
Or, you can add ‘t’ to the Asterisk Trunk Dial Options for the trunk to the NEC. The NEC user can dial *2 and hear Asterisk dial tone, dial the transferred-to party, speak with him if needed, then hang up and Asterisk will bridge the two external parties together.
These options also differ on what gets logged on each system, how calls can be recorded, etc.