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Usually because of the way the house is wired up. Do you have the Phone plugged in to the primary socket and the modem in to another socket somewhere else in the house? If it is wired up incorrectly, when the phone rings it will disconnect the other outlets.
You could get an electrician around to rewire the sockets so this doesn't happen.
The inline filter may not work. At my old house, because of the way the wiring was originally done, when the phone was picked up on the primary socket, it physically disconnected the other sockets. An inline filter doesn't help in this case. The wiring needs to be changed so that the other sockets remain active when a call is in progress at the primary socket. Note: in this case, if you switch the modem and the phone around so that the modem is on the primary socket then the problem is also solved but the primary socket may not be in a place where the modem can be connected easily.