Answer to the question: Will the battery for the Plantronics CT14 work in my discontinued Plantronics CT12?

Will the CT14 battery work in a CT12 remote?

Unfortunately they're not compatible and interchangeable.  Although the Plantronics CT12 and Plantronics CT14 systems look alike, they do use different batteries.  The CT14 battery ( part # 81087-01 ) is much smaller than the CT12 battery (part # 63421-01 ) , not just in physical appearance, but also in power specifications.  The much smaller CT14 battery does have the same plug as the CT12 battery and can physically "plug in", however it does not have enough power to even turn on the CT14 remote.  The specs are as follows:

CT12 battery - DC 3.6v, 800mah  /  CT14 battery - DC 2.4v, 600mah