I subscribe to ISP that gave me internet, cable TV, and phone service. ISP gave me a fiber optic modem router combo which came with RJ45 port and RJ11 port.
I plugged in my PC/STB into the RJ45 port for internet and cable TV. I also plug in my analog phone into the RJ11 port. Basically everything is plug and play and no settings required on my end.
Today I look around at the modem web interface and found that they have SIP account information inside it.
my guess is no you can’t use it. most isp’s these days run their “analog” lines over sip. the sip account you are seeing, is, i suspect, the account they use for your analog line and phone number
Your ISP tech support should be able to answer this, eventually. You might need to ask for Tier-2 support. Remember that the answer may well be “No”, but there’s nothing keeping you from spending a couple of dollars (or local equivalent) a month and setting up a VOIP account.
In most cases the answer is no.
They usually provide their own VoIP service in a separate VLAN (and isolated, private IP subnet), so it is not accessible from your LAN port and from the Internet. Even if you will manage to bridge one of the LAN ports with the Voice VLAN they will not give you your SIP account password. This is a typical worst case scenario. Sorry
