[Blind-sysadmins] Phone splitter boxes

Hi. I am helping someone using an IP desk phone with an RJ9 port. They want to use a single headset with a microphone, but have the output from the phone come in through one ear and the output from the computer screen reader come in through the other. There used to be a box that did this, but they are no longer available. The user can't use a softphone for various reasons. I'm thinking a mixer may be the only option but does anyone have anything they've used recently for this purpose? Thanks. Andrew.