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.
What is best way to tie the two buildings together and they would like to use the digital sets at each site and not IP phones.
Can anybody make a suggestion for this.

You will need to go IP handsets due to the distance.
This would be a cost effective solution compared with upgrading the CIX100 to a CIX670 and installing 2 x RRCU cards and having a remote 670 cabinet running over a fiber link just so you can run digital DKT handsets at the remote location.

Is there a fiber connecting the two 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?

Thank you for your quick response. There is a multimode fiber between the two location so I am thinking a MIPU card with 24 licenses is the way to go. That should be the best solution in the long run. Can you confirm this is the best option for me.

Once again thank you for your help.

I would add the MIPU24 card and 24 x IP endpoint licenses if I was going to be doing the job.
(Don't forget the MIPU24 will use 2 x 16 port slots so might be easier to supply 2 x MIPU16 cards)

The operation of the IP handset is the same as a Digital so the customer won't see any difference between the 2 types.

Unless I was going to do a 24 channel SIP trunk, I would opt for 2 - 16 channel MIPUs for better port density.

Even though Slot 3 on a CIX100 was designed for a 24 port T1 card, the I&M manual says that you still need 2 slots for an MIPU24

Remote cabinet and get a bunch of 4-port basic licenses for now and the future. You can find used Toshiba parts for cheap. Toshiba is going to disappear soon, including licensing, so I would stay away from IP phones. What happens if they need a few more IP phones in the future? No licenses. No phones.