Forward calls disconnected

hello.

I have 1 extension that is being forwarded to an external number trough a SIP trunk.
the call gets forwarded it rings and they answer but once they finish answering the call gets disconnected, I can heard them answering the phone.

I’m using this to forward the Spanish calls to a call center.

If I dial directly from one extension to the extension to get forwarded the call connects with no problem 100% of the time.

sip phone => extension => external (works well)

Incoming sip => ivr => => extension => external (fails 99%)

Can someone please give me some directions to find a solution to this problem.
it happens 99% of the time the other 1% the calls can get connected and they can talk.

This is almost always a network configuration issue. You did not tell us anything about your configuration or what software you are running however I assume your FreePBX system is behind a network connection using NAT.