Make sure your system has a fully qualified hostname. If properly configured during the installation process, the following lines should be in your /etc/hosts file:
127.0.0.1 localhost.localdomain localhost
<your sipX server IP addr> sipx.your.domain sipx
Test hostname and domain name settings:
hostname (outputs your fully qualified name)
domainname -f (outputs the fully qualified name)
domainname -d (outputs the domain name)