A telephone network access device switchable between a VOIP service and a PSTN service includes an RJ-11 port, a voice splitter, a first relay, a second relay, a SLIC chip, a CPU, and a ring detection unit. The ring detection unit outputs a first switch signal in response to detecting a ring signal of a PSTN communication. The second relay connects the telephone to the VOIP network in response to a power on event. The CPU controls the first relay and the second relay to connect the telephone to the PSTN network in response to the CPU receiving the first switch signal, and controls the second relay to reconnect the telephone to the VOIP network when the PSTN communication ends.