Although you cannot assign a static IP address to the base station, you can fix it to where it will not change. You can go into your DHCP leases and reserve the address instead. This should insure that the DHCP server does not assign the device a different IP address in the future.
Regarding the PSTN Gateway, you can assign it a static IP address by using the web interface for the device. To do so, go to the current IP address of the PSTN and add the port number of 9999 (ie: http://192.168.0.148:9999). If you haven't changed the password, you can login with admin and admin for the username and password. Once logged in, click on the Network tab. In the WAN SETTING section, you can select STATIC from the Connection Type list. Once you have setup your STATIC settings, click the Apply button at the bottom of the page. Once you change the IP address, the next time you access the web configuration utility for the PSTN gateway you will need to use the new IP address you specified with the PORT of 9999.
Good luck...