OK, a little clarity -
What you have from Comcast is referred to as an eMTA (enhanced or embedded multimedia terminal adapter). It is a cable modem that is also using PacketCable as a network delivery mechanism for multimedia type services such as phone service. The delivery of the voice over PacketCable uses NCS in early versions of PacketCable but is now turning towards SIP in v2.0. (I built and repaired MTA, eMTA and CMTS systems for a rather extended time in my past life).
Despite what Comcast told you, your understanding is not quite correct. -
Inside the eMTA the NCS/SIP data is converted into an analog output, which is what you can connect your everyday analog phones to. It is a digital to analog conversion when the signal is sent to those analog ports and on the reverse side it is converting the analog signal from the phone into a digital signal for the soft-switch via a CMTS system where the call handling is processed back at Comcast.
This above conversion is what a HT does. It takes a digital SIP/RTP stream and coverts it to an analog signal so you can connect an analog phone, PBX or fax machine. And FXO device does the opposite and converts an analog signal into a SIP/RTP stream. FXS provides a ring voltage, FXO accepts a ring voltage.
You cannot take an IP phone and plug it into the Ethernet ports of the eMTA and expect to have instant service. The phones need some programming to tell it who/where the soft-switch is located, and some credentials by which the phone will know how to take a call and the soft-switch will know that you are actually authorized to use the service. Comcast will use a provisioning service by which to setup their eMTA with this info, but not your IP-Phones.
However, most cable companies do offer SIP trunking service and some may now have hosting. If this is the case, then you may be able to subscribe to such a service and use the phones directly, but most of the time they impose a limit on how small a service they will accommodate. In my area, the local cable company will not accept direct SIP trunking unless a minimum of 7 calls paths are subscribed to. If lower than that, only analog.
You can also consider moving your numbers (porting) to a SIP trunk provider. They are more lenient and you will find many on the forum who use IP-Phones connected to such. This would use the Ethernet of the eMTA to transport the signal between your IP-Phone and the SIP trunk provider.
The final option is to use an FXO gateway in conjunction with a switch. This would take the signal from the analog ports of the eMTA and then convert them into SIP for connection into the SIP phones (via the switch). However, there may be some pitfalls in this approach as it depends on your expectations of what is to be accomplished from a call handling perspective (other than I want to use a IP-Phone). As there is no PBX, there will be quite a bit of limitation as to what can be done and the configuration might be somewhat complex. A PBX is typically used to take multiple signals and then manage how sent to their destination with various call handling features. As you have two lines, a PBX would take both lines in and then send 1 or the other or both to a phone. As there is no PBX you have the issue of two lines and two phones and presumably you want to be able to accommodate both lines on both phones which may mean using multiple profiles in the gateway and multiple accounts in the phones. Other than simply wanting to use IP-Phones, I am not sure the gain that is expected over a dual line analog phone(s). However, that choice is yours to make.
Sorry, for the long explanation, but wanted to make sure you understood the options.