Some landline phone calls are not getting through

Some incoming calls to my Xfinity Voice landline are not getting through. Here's what happens: My landline phone will ring once (and only once). The caller ID display shows "Incoming Call." But when I pick up the phone to answer the call, all I hear is the dial tone. Evidently the call is being dropped or blocked before I can answer it. My Xfinity Voice call blocking settings are set to allow all calls regardless of "spam" probability (high, medium, low), but some calls still do not get through. This does not affect all incoming calls -- maybe only about 10 to 20 percent -- most calls come through and can be answered normally. As far as I can tell by looking at my Xfinity Voice call history log, the affected calls are from non-local (i.e., different area code) numbers, most often belonging to a company or business. I have contacted Xfinity Voice customer support agents on several occasions, but they've been no help whatsoever. All they can tell me is that there's nothing wrong with my phone line or service settings. One rep said the calls must be getting blocked as outgoing calls on the caller's end, which doesn't make any sense. One other thing I should mention: I have my call forwarding options set so that all incoming calls are supposed to ring both my landline and my cell phone (Apple iPhone 17) and usually this setup works well. As I said above, however, the problematic calls ring my landline phone only once and are never forwarded to or detected by my cell phone.