A box labeled "Network Interface" is usually for telephone landline connections. This might be for a single line, two lines, or multiple lines. The Network Interface on the exterior of a structure was introduced to define the point where the Telco's responsibility for wiring ends.
When "telephone ready" was a feature or option on many RVs the feature meant having a network interface installed, for the phone company's connection. This is not a common feature now that many have abandoned landline connections, and you will find fewer and fewer parks offering the service.
Tom Test
Itasca Spirit 29B