Why did customer who call to ipphone have to wait more then 20 sec ? How to solution the problem ?
There is far too little information here.
If you are contacting the iPhone over GSM, it may take that long for the paging channel slot to come round in the cellular network, which is something that is totally out of your control
However, I suspect you are not using the cellular network, in this case in which case you need to tell us how you are connected, and you need to provide detailed logs to show exactly where in the call the delay is occurring. You will probably need to supply the protocol log for the protocol you are using to communicate with the phone.
Typical reasons for delays include broken DNS servers that do not respond at all.