ZAP Channels Blocking and unavailable with PRI "Call&qu

I have a problem with some PRI channels wich are becoming blocked (and therefore unavailable for use) with the PRI Flag “Call” being set on. I works fine for hours, but at one moment, the “Call” flag (p->call in the PRI structure) appears to be being set and not released on hangup or reset. Once blocked, the channel remains unavailable for in or outbound dialing.

Even with no “channels” in operation (show channels) the blocking PRI Call Flag can be seen in the PRI structures (zap show channel $n)

The only way I find is to restart Asterisk to clear the flags…
Does any one have any other better solution to resolve this ‘bug’?