Using iPhone on VoIP landline?
It is not possible for an iPhone to answer calls that are being routed to an analogue SIP port. Depending on your VoIP provider, however, you may be able to install a SIP client, e.g., Acrobits, on your iPhone and configure it to work with your VoIP providers servers.

It's unlikely that your VoIP services support the same number to be configured on both your SIP modem and a SIP client on the iPhone, so it would be a case of either your modem is configured to provide VoIP services for your phone number or the iPhone is. It's unlikely that you'd be able to register two different clients to handle the same phone number.