Trunk IAX o SIP

I have 2 machines connected with IAX and 1 with SIP.
The calls arrive from SIP, and through IAX go to HOST.
I have this problem, it is a possible have 1 CALL simultaneous.
What is the problem?
O where i need see for resolve the problem?

Assuming “only possible to have one simultaneous call when it originates over SIP”.

Is the second SIP call being offered? If not problem is upstream.

Is the call being rejected? If so, before starting the dialplan (please provide sip.conf sip traces and verbose logs), by the dialplan (please provide verbose trace and the actual dialplan code, or on the outgoing leg (please provide IAX configuration and IAX debug logs).

I see the log enabled the debug to IAX.
I used ASTERISK 1.8

Asterisk 1.8 is past end of life.