You can take your VoIP service with you when you travel. When you get to the hotel or other location do the following:
Connect the WAN (Internet) port of the ATA to the Internet connection, i.e., a cable or DSL modem or the jack in your hotel room.
Access the router configuration. Ensure the router is set to provide DHCP service.
Ensure that the router is set for the proper service, i.e., cable or PPPoE. Hotels will likely have a T-1 line or other high bandwidth connection, which is configured like a cable connection.
Ensure the MTU, if present, is set to 1492 for DSL or 1500 for cable or a hotel connection.
Connect your telephone(s) to the telephone ports on the ATA.
If you are using a laptop, connect it to one of the Ethernet ports on the ATA.
If you have an ATA router, and if you also have a router at home, you can connect the ATA to your existing network very easily.
This technique should work in most situations. If your phones will not connect or if you have sound quality or reliability problems then you may need to purchase an ATA without a router plus a separate router that you use with the ATA when you travel.
Connect the WAN (Internet) port of the ATA to one of your router ports.
To configure the router, connect a computer directly to one of the ports on the ATA router. You can not do this through your network.
Ensure DHCP is enabled so your telephone connection can obtain an IP address.
Set the router to use a cable connection, even if your ISP uses DSL or fiber. This will give you a direct connection to your local network.