Yes...it is possible...im not sure how...i just know that it is because it happend to me but i had picked up my phone and heard her conversation but she couldn't hear me...
Cordless phones use radio waves to communicate between the base and the handset. If you neighbor's phone happens to use the same frequency/channel at the same time there could be a connection between both conversations. It happened more often with older models, but shouldn't happen with models made in recent years. People with radio scanners may be able to monitor the frequencies that some cordless phones use, but that too is less likely to happen with newer cordless systems.