It could be as simple that before there was no queue and all calls were handled by a human or a busy tone. There was no way to know how many calls never made it through.
If newly implemented, then callers may be surprised at how the phones are now answered. Perhaps they "hate" the idea and hangup and think it possible to avert the Q by simply dialing back in an hitting "0" for a operator/receptionist.
How long is the queue set and is it possible that it may be too long? Do you announce so folks have an idea of where they are in the queue?
Who actually gets directed to the Q, customers, vendors, etc.? Vendors/sales types will typically only stay a very short period before moving on.
Is the greeting message clear about the Q's purpose and how it works.
Get some of your friends and family to call in and get their feedback on how they view its friendliness and functionality and more so about at what point and why they may have jumped off.
It could be, that without any prior baseline reference, what you are seeing is typical for something new and that as time goes by and people get used to it, the drop rate may go down.
I have seen it on occasion where a client gets cold feet as for the first time he now sees that people don't always wait and he/she will want to revert. This typically occurs when (previously) they were using a limited number of PSTN lines and no matter the situation could only handle no more calls than the number of lines. Then, they buy 3CX and go SIP and can handle more calls, but the staffing remains the same, so the need for a Q arises and, like you , see callers abandon. I have to remind them that before, the caller would get a busy and not being able to get though could conceivably take their business elsewhere. At least a Q allows the caller to have the opportunity to be answered, stand in line and wait their turn rather than being out-right rejected from the start. The key is to watch the stats and make the decisions needed to make the Q work for you.
You may need to make some subtle changes to target the Q and then to find the sweet spot so as to best retain callers in the Q. It may also mean that added staff is required, but if it means added business/opportunity rather than a busy tone, never to be heard from again, this should be a welcome tradeoff.
