I just got FiOS a couple of days ago, and I notice when I hang-up a phone call, either my phone will ring and it would still be the party that I hung up from, or if I pick up the phone quickly the other call is still there. I notice this especially if the other party has not hung up yet.
While I cannot directly answer the question, have you tried checking to see if the problem occurs with only specific phones or if it occurs while plugged directly into the ONT's test jack (if Digital Voice uses that)? At my home we have this problem occur occasionally, though I am still on Copper service as FiOS isn't around here. I've noticed it happens the most when a phone is hung up in an odd way, making it seem like we're "Flashing" the call. When this happens, the phones all ring indicating we left a call running. Sometimes though I have noticed calls not hanging up. Those are far and few, but when we hang up the phone and wait 15 seconds, no phones will ring but when we pick up the phone again the call will still be connected. Not sure if the DSL might have something to do with it or if I need to get the field tech down the road to replace the line card again (it went bad 6 years ago preventing us from receiving incoming calls properly), but the line doesn't act up otherwise.