Degug SIP registration problem

Hello I have SIP server running under FQDN domain name (ie. [login to view URL]) Most of softphones and hardware ATA's can register with no problem when they use domain name. The problem is that some softphones and ATA's could only register using server IP address. Best example is very popular softphone X-Lite. When configured to use IP address it works perfectly, when domain name is entered it could not register to server. Server is running Yate 3.3 softswitch, but exactly the same problem was when I've used asterisk PBX. Server has also configured DNS SRV records but it does not help. Your tasks: - discover problem with configuration (remotely, I can not give access to this server, it's production enviroment) - provide instructions how to correct this problem I will accept your work as finished and make a payment only when provided instructions wil solve this problem (when X-Lite softphone will be able to register to my server using domain name in format [login to view URL] instead of IP address). Test will be made from my home computer.