I agree with Brian that you likely to have a better outcome, if you visit the store. I returned a cable modem at that exact location, and learned that I needed to register my new cable modem once I got home via the Internet.
It's much better to walk in then to try to call. If you call you might end up with someone who doesn't understand English well, there for doesn't understand what you want. The face-to-face approach is way better.
There is no direct line to them. You need to call the main number
They don't do business over the phone. Contact Xfinity and they will troubleshoot your issue or suggest you exchange/replace equipment at the store, via UPS or whatever suits your needs.
Call the xfinity 800 number
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