I have problem in queue and call waiting service!

hi , ive enabled recently the queue service ,

ihave some issue related to this service,

now im using a mix of grandstream phones.
now , i want to configure that if a spesific user was busy and i transfered to him a call , i dont want my call to be lost becuase the user was busy.

now , if i didnt enable from pbx "call waiting service " the my transfered call will be lost ,.

but if i enabled it , the busy user will recive the call in another line .

now i could solve this issue . …but …

the problem is , as soon as im enabling the queue service , my phones will still have an incoming calls during calls becuase i enabled the call waiting service and i think that this is an annoying issue.
my question is how to enable the call waiting service , and when someone on queue waiting ring only and only if the busy user end his call ??
"""i mean that i dont want the person who is on queue waiting to ring my phones during busy callser and permit ringing during transfering “”

canany one suggest another solution ?

how to solve this issue ???