I am currently on nbn™ Hybrid Fibre Coaxial (HFC) with phone line connection for our office.
Would like to know whether I can still make and receive phone calls if I purchase a CISCO SPA525G2 IP Phone and plug that directly to the LAN port of the Optus router?
As the current phone line is already on VOIP setting so I was assuming it will work.
Optus will not give you the SIP credentials so you will not be able to configure the cisco to use it.
One unknown is that even if you got the SIP credentials the Optus router may not be able to properly pass on the VoIP signal to the LAN port because the router is configured to handle the call and there is no way to turn that off.
Thanks for your comment.
Would you know if it will work by connecting the phone cable to the "Phone 1" socket on the router instead of the LAN port?
Phone 1 is an analogue phone port just like the old Telstra/Optus wall sockets, so you can't just plug an IP phone (that is expecting a LAN data connection) into that port.