Bridge mode is a service that many routers have. The purpose for using bridge mode in the realm of VoIP is because we assign IP addresses directly to each physical device. If the network is setup to have two routers or has double NAT it can cause issues inside the network such as one-way audio because when the traffic returns the packets don’t know what IP address to go to. Another reason is that you may have a router modem combo that cannot disable certain features like SIP ALG that can interfere with VoIP traffic.
Your solution depends on the type of network you're designing. If you have only one router on your network and it is set to bridge mode, your network (wired or wireless, or both) will be active but will not be open to connecting to your internet access. If the single router is not set to bridge mode, your network will be able to be connected to your internet access.
When your network will contain more than one router, select bridge mode for each of the additional routers if they will be connecting to a router or other network device that is already using DHCP and NAT to provide IP addresses to computers on your network.
You can use any one of the routers to be the DHCP/NAT server as long as the others are setup in bridge mode. You will usually want to choose the router with the greatest serving capacity to be your DHCP/NAT server.
What bridge mode really does is to allow additional routers to connect to the network without conflicting with the DHCP/NAT server. Additional routers may extend the network's wireless range and/or enable additional wired connections for clients and devices.
In simple terms turning the router to bridge mode effectively turns a router modem combo device into just a modem. This makes it so that you can use your own router on a network that the isp needs to communicate via their proprietary devices. In most cases, you can set the device into bridge mode yourself and it will function fine. However, occasionally there may be a case that the router will not lock the settings that you chose. For instances like this, it is recommended that you call the isp who provided the device to lock those settings.