Transfer a call from a queue drops after 4 rings

i dont know if sunject is correct but i have this situation.
a call goes to a queue 800, and there one of agents (for example 21) tranfers the call to extension ‘10’.
what happens is that after about 4 rings the calls ‘returns’ to the agent.

the same doesnt happen when ‘21’ transfers internal call to ‘10’.
‘10’ is an exmaple. this happens with every transfer from queue to extension,
i dont know where to search and what to search. is it queue issue? or agents transfer issue?