We have the next problem:
Two users speaks on 7942G phones through Call Manager. If first user speak something and second user speak some words at the same time, then each user hears interrupted voice of another. At 7940/third party devices the same problem.
Can it be fixed on Call Manager?
RTP is sent directly between endpoints, unless you are using an MTP/TRP which would be weird for an internal call, so, no, this is not something you fix in CUCM.
That sounds like some network issue you're having with the RTP, or that you're not using full-duplex