I've used a good quality harris ts21, polycom soundstation and an at&t feature phone 864. All same result. The CSU I used was Adtran ACE which is a pretty basic unit. NTAC did not suggest a particular make/model of csu.

I don't know if I can get into the Edgewater 4550 or not. I've found manuals for it and can see there is a T1 configuration screen. But it isn't mine and risky perhaps.

I got the customer a pots line so they can use their new Polycom but I am rather disappointed the sl1100 has this problem and I don't know if it has happened to other people with PRI on the SL1100.