If you are speaking about the address of your internet connection, you need to speak with your internet provider.

If you are speaking about a static address for something within your home/office network, it is entirely dependent on the device.

Some telephone adapters can't be configured with a static address and are dependent on DHCP. If that's the case your router should be able to be configured to reserve a specific address for the MAC address of your device.

It would help if you provided a more detailed description of what you would like to accomplish.