This is a common problem. It appears that some combinations of phone/adaptors and DHCP servers do not communicate the Primary/Secondary DNS servers settings correctly.
The solution is to manually set the Primary/Secondary DNS server in your adaptor configuration.
For the xlite software this setting can be found under the System settings | Network option in the configuration menu.
For the Sipura adaptors/phones this setting can be found under the System Tab in the Optional Network Configuration section.
If you do not know what your Primary/Secondary DNS server addresses should be, please contact your ADSL/broadband/cable provider who will be able to provide you with the appropriate IP addresses.
In addition if you are using a Sipura 2000/3000 adaptor also check the following settings are set to "YES":
Handle VIA received:
Insert VIA received:
Handle VIA rport:
Insert VIA rport:
These settings can be found under the SIP Tab in the NAT Support Parameters section.