PSTN blind transfer to extension

I have a problem; any calls from DAHDI analog trunk (PSTN) blind transfer to extensions, PSTN caller can’t heard if the extension is ringing but other calls not coming from PSTN blind tranfer to extension is OK and no problem.

By the way I’m using Sangoma A400: 2–24 Port Expandable Analog Voice Card

any solutions or suggestions is highly appreciated…