One Inbound Call -> No more outbound calls

I am still having the issue that once I receive an inbound call on one of my ZAP channels I can no longer make calls from that extension any longer...

My configuration:
-Rhino card with FXS module that is connected to a two line analog phone
-Standard setup; fully patched
-Can make calls on both lines before receiving a call
-No error message in any log
-Specified nat=yes, external IP and local network in nat config file

Basically, it seems that my Rhino card goes into a dream state once it received an incoming call. I mean I still get a dial tone when I pickup my handset, but it remains a dial tone no matter if I dial an internal or external extension.

Is it possible that I got a faulty Rhino card?

Any help would be appreciated!

Thanks,
Jens