It's very common for ISPs to share common lines and such, especially when they are servicing the same physical location. It's therefore critically important to ensure path diversity, where the lines out of your ISP follow diverse paths to other routers. A common method for path diversity in smaller centers, for example, uses a fiber connection for the primary path and a very fast cellular connection as a failover. Use of both a fiber ISP and a different cable ISP can also lead to path diversity.