Boy, the comment about the smart card jogged my memory. Here is what happened to me, although it was on a standart T1, it might have some relavance. I have two SX2000 linked and from day one I would have intermittant loose of sync and the T1 would fail. When telco put the Tbird in they would see esf even though the circuit was designed as ami. The T1 was stable, but wierd, for 2 years. Then it really failed. Same thing. When the pbx's were operating, telco would see esf when they monitored it (I saw it on their Tbird). If you disconnected one of the sk2k's, all of a sudden the circuit would go ami. They would look at me and say, "see your machines are putting out esf" and i of course would say, 'that's impossible'. Loads of finger pointing and finally I got it escaladed to the point where I got 2 telco techs and they went head to head with the Tbirds.
So here we are, 2 pbx's up and running fine, 2 Tbirds at each end and both Tbirds showing esf and dtstat showing errors. Pulled the the T1 at the far pbx and both Tbirds show ami. I disconnect the near pbx and the circuit is ami. "see, your pbx's are esf', they say. OK, I say, put the far end Tbird to send a pattern and make it ami. "What the....?" says the near end tech. 'I see esf'.
Turned out to be a bad telco smart jack at the far end. The smart jack was converting the ami to esf and then back to ami before sending to the circuit. Of course it was not doing the last conversion and was sending esf. Changed the smart jack and its been perfect ever since.
Moral of the story. When telco says that a T1 is just a pipe and we don't do anything but pass the data, don't beleive it.