Today someone asked me if he can make calls using a regular landline phone connected to a VOIP network without using a VOIP adapter, just by using an Ethernet to Telephone line adapter, at his work he has access to a regular landline in addition to the VOIP service via the same ethernet cable is this possible? sorry if this is too obvious I am not an expert on this topic.
No. VoIP over Ethernet is so vastly different from POTS that it's absolutely impossible to connect without a SIP gateway (which might come as an "Ethernet to telephone adapter" but it requires power and configuration).
However, it's quite possible that "someone" has access to both, a regular landline and a VoIP service, using the exact same IP phone. That phone is connected to a PBX and the PBX simply uses different types of trunk lines (and includes said SIP gateway).