Normally the system will find the correct DNS servers by looking though the NIC Adapters you have configured.
For some reason global call occasionally has trouble resolving names. This is particularly important when using SIP.
If you have this trouble, you can manually add the ip addresses of your name servers to:
Separate additional name servers by a comma.
http://www.dialogic.com/manuals/docs/globalcall_for_ip_hmp_v10.pdf Section 12.3 for more information.