user_urph6a Thank you for taking the time to reach out to us here on our Xfinity Forum. I'm sorry to hear that you aren't able to get your phone working. Can you clarify the type of modem you are using for me, if there is a stand alone router also in use, as well as what troubleshooting steps you have already attempted?
I have the Xfinity XB8-T modem with built in wifi. I have no other router installed.
I have reset my phone adapter several times with no change. I have contacted Xfinity and they say the modem is working fine which is true but my phone adapter will not register with this modem.
user_urph6a Have you attempted to plug the phone line directly into the back of the modem into the TEL1 port?
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My phone is not with Xfinity, it is with VoIP.ms. Plugging a bare phone into the modem would do nothing. If I could get into the modem somehow and change the settings I might be able to get my Cisco adapter to work
@user_urph6a - It sounds like you're dealing with a classic compatibility hurdle between a modern gateway and a legacy VoIP device. The Cisco PAP2T is a very old (nearly vintage) Analog Telephone Adapter (ATA), while the XB8-T is our latest WiFi 6E gateway. The issue likely isn't that your modem is "broken," but rather that it is blocking the specific type of traffic your Cisco adapter needs to talk to the VoIP.ms servers.
Why it’s not working The XB8 modem has a built-in firewall and a feature called SIP ALG (Application Layer Gateway). While intended to help VoIP, it often corrupts the data packets for third-party adapters like the PAP2T, preventing "registration." Additionally, since the PAP2T is an older device, it may struggle with the way the XB8 handles DHCP (assigning IP addresses).
Things you can try right now 1. Change the Ethernet Port The XB8 has one port with a red stripe (the 2.5Gbps port). Sometimes legacy devices like the PAP2T have trouble negotiating a connection on high-speed ports. Try plugging the Cisco adapter into one of the other three standard Ethernet ports.
2. Access the Gateway Settings You mentioned wanting to change settings. While we lock down many features, you can still access the local admin tool:
- Look for Firewall settings. Try setting it to "Low" temporarily to see if the adapter registers.
3. Disable SIP ALG (If possible) In that same 10.0.0.1 menu, look under Advanced > Device Discovery. If you see "SIP ALG," disable it. Note: Many Xfinity firmware versions have removed the toggle for this, meaning it's "on" by default.
The "Bridge Mode" Solution If the steps above don't work, the most reliable fix is to put our modem into Bridge Mode.
- The Catch: This turns off the XB8's built-in WiFi entirely. You would need to buy your own standalone WiFi router.
- The Benefit: A personal router (like an ASUS, TP-Link, or Netgear) allows you to easily disable SIP ALG and open ports (Port Forwarding) specifically for your Cisco adapter.
A Note on the Cisco PAP2T The PAP2T was discontinued many years ago and does not support modern encryption (TLS). If you find that the XB8 simply won't let it through, you might consider upgrading to a newer ATA like the Grandstream HT801 (recommended when searching online). They say it’s much more compatible with modern "smart" modems and works seamlessly with VoIP.ms.
Please let me know if this helps!
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