I have a school with an existing CIX 100 cabinet and they have built and addition school building which is at least 350 meters away. I do not believe the Toshiba digital stations will operate one one pair at that distance. Using two pair is not an option as they have 16 new phones and only 20 pair ( 5 Cat 6 cables ) between the buildings.
Per the I&M, the maximum distance 1 pair or 2 pair is 303 Meters. You are correct about the distance, but that second pair isn't going to help.
I won't have much addition info to offer since you already got replies from a couple of Toshiba experts, but in the log run I think IP may be the last available option. You could add a second CIX processor and QSIG over IP, but that would not be my first choice.
Either that or use standard analog phones, and grow tto a CIX670, as previously mentioned
Is there reason why you would not want an IP solution at all?