Client insists they used to have an Avaya 4-line phone that worked with their ACS system. They want a "new" one.

Is there such a beast and are they any good?

Any other cordless recommendations? They don't want wireless headsets.

https://support.avaya.com/css/P8/documents/003761537

work like any other system cordless imo

They probably had one of the TransTalk cordless phones:

https://www.transtalksunlimited.com/

I believe the 3910 has been discontinued. The Transtalks (long discontinued) have issues with interference causing loud clicks and pops.

-Hal