Metro 100 off-line 2M circuit is connected to enterprise voice switch

Metro 100 off-line 2M circuit is connected to enterprise voice switch, and it is disconnected when dialing an internal line.

The two open circuits between the different sites of users A and B use Metro 100. Two Metro 100s are connected to different computer rooms, and the transmission network composed of OSN 7500, 3500, Metro1000, etc. is used in the middle. A 2M point is connected to the Ericsson small switch (using operator fixed-line), point B out of 2M connects to enterprise branch exchange (using telecom fixed-line)

The internal lines of users at points A and B (this circuit is No. 1 signaling) will be disconnected from time to time, but they are all normal when they dial outside (this circuit is No. 7 signaling). Set up a platform in the online room at point A, install a new enterprise switch, and dial internal lines to point A and point B respectively. All are normal, so the transmission failure is eliminated. However, the switch manufacturers of both parties also eliminated the equipment problems of their own manufacturers, and then replaced the point B equipment with Metro 1000, and the failure phenomenon was eliminated, but so far, the cause of the failure has not been understood?

I ask an expert to analyze!

Thanks