MFCR2 call normal until suddenly audio goes noise

I'm having this problem:
In my workplace, I was tasked with replacing the functioning asterisk-freepbx PBX running old versions of debian and asterisk, for a new asterisk PBX w/o freepbx.
The PBX had a couple of OpenVox analog cards, which didn't give me any major trouble, and a digital D210P card, connected to a MFCR2 line from Claro(telmex). I'm in Argentina. The digital lines were working.
When building the new pbx, I took the digital card from the old one and tested it in the new one, I got it working, tried some calls and everything was good, apparently.
Then, when we moved the new pbx into production, people started complaining the calls would cut with noise, or some other people thought a fax machine or modem took over the line, because the calls would connect, the two parties were able to talk, but after some time (not a fixed amount of time, most of the time about 1 or 2 min, sometimes more) both parties stopped listening to eachother but a high pitched noise, similar to acoustic feedback or the screech of analog modems when they try to communicate, but the noise was digital in nature, not analog, not feedback, it was constant in volume and didn't alter when speaking into mic, or over time.
To test without having the other lines drop every time we had to reconfigure dahdi or restart the system, we moved the card to a test box (the hardware from the old PBX being replaced, but a new disk with the same system as the new one, devuan jessie).
We got the card and the mfcr2 line working, made test calls and the random noise was still present.
We checked dahdi_tool for IRQ misses, at the same time the noise appeared, and nothing: (0) misses.
We disabled echo canceling, with no success.
We tried changing the balun, with no success.
We tried using the same version of DAHDI as the old working box (2.4.1) with no success.
We used dahdi_monitor to check the levels, and when the noise appears, the TX still shows the level of the call sent by asterisk (for example, if we let the call play MOH forever, we see the MOH levels on TX), but on RX we see the level of the noise received (2032 is the value of the noise, always constant, doens't matter of the other party is silent or yells in the mic, when the noise starts it continues always constant).
We run the dahdi_test tool and we always get like 99,999%
We tried to stress the CPU during a call to trigger the noise, without success.
We monitored the mfcr2 channel log during the call, but when the noise appears, nothing comes up... like the call was still normal, in a sense that's true because the call is still stablished, but both parties hearing a noise (that I dont know where it comes from..). When one of the parties hungs up, then I get the event in the log file, like normal.
We used the same configuration parameters as the old working PBX, if you need them I'll paste the system.conf or chan_dahdi.. but I don't think they're the problem, because we can get all the channels up and running.

Now in the test box im running:

pruebatrama*CLI> mfcr2 show version
OpenR2 version: 1.3.3, revision: (release)

pruebatrama*CLI> dahdi show version
DAHDI Version: 2.4.1.2 Echo Canceller:

pruebatrama*CLI> core show version
Asterisk 13.11.0-rc1 built by asterisk @ pruebatrama on a i686 running Linux on 2016-08-04 17:43:00 UTC

I'm running the old dahdi version because it's the same the old working pbx had running. It's the dahdi distributed by openvox and installed according to the card manual:
www.openvox.cn/pub/manuals/Release/Engli...AHDI_User_Manual.pdf
We initially tried the current 2.11 release but it didn't work at all. The previous 2.10 did work (with the same noise problem), but then I replaced it with the 2.4 to test.

Let me know if there is some information I left out...

Thanks in advance.

Julian.