Receiving and placing more then 2 calls from one sip account

Hi people!
I have installed Asterisk where a grandstream telephony adapter with 2 FX ports is connected through.

At one port my cordless phone DECT station is connected through.

If a call comes inside a cordless phone and I answer the call, I can’t place a call with the other one.

What I want:
I like to receive and place with 2 cordless phone through the DECT station connected to 1 FX port of the grandstream module at the same time.

How do I make it possible that I can receive through one FX port more then one call and place more then one call?!