This issue is a very old and have been ignored several times in this bug tracker. Custom channels do NOT work as intended and is probably the soul purpose of lacking vanilla COOP public servers with massive teamplay. So this issue is HUGE.
What is it:
If you are inside a vehicle. You are unable to hear messages being broadcasted on the custom channel. If however, the man you are talking to switches another channel and speaks, then switches back. He is able to send 1 message to the man inside the vehicle. Some sort of index error is occuring when vehicle channel is added to total amount of channels i think.
This is shown in detail here: https://www.youtube.com/watch?v=Oc6cF5P6iWk {F23327} {F23328} {F23329}