Transfer call via ISDN

I have two pbx: asterisk and panasonic tda100. It’s linked with E1. For panasonic E1 is FXO, also for panasonic E1 is remote pbx with phone pool 70x. local phones are connected to panasonic with analog and one sip phone is connected to asterisk. External sip lines connected to asterisk.

When I recive external call (from sip line) asterisk running dial(dahdi/g0/601). After answer (from panasonic) I tried to transfer call to sip phone (703, back to asterisk). Transfering was well done, but I had 2 busy lines on E1. Is there any issue for free lines, when call transfers back to asterisk from panasonic.

Please, help!

I found a few posts about ECT. Is it solution and how I can configure it?