Project Fi Pixel phones are being blacklisted as stolen devices according to one user who had recently purchased the device and has now run into this issue. For clarification, the Pixel and Pixel XL devices that are coming up against the blacklisting issue are resold phones which were originally purchased through Project Fi, so it isn't happening to random customers with either device who still use them on the Project Fi network. In this particular case, a user named Kyle who has posted about the issue on HardOCP, had purchased the device from Swappa with the intent to use it on T-Mobile's network. After a month or so of use following the purchase, the phone was limited to emergency calls only which seems to have been in effect since yesterday morning.
According to the user, who had talked with both Swappa Support and Google Pixel Support, the phone didn't appear to be blacklisted, with one Google Pixel Support team member stating that a message the user received that was referencing a stolen device, causing the blacklist, was just a "spam message" and they were told not to worry about it. The issue according to the user is that it isn't a spam message and the phone certainly is blacklisted.
As it stands the user was told they would have to be put in touch with the Project Fi support team to continue with the issue and that an email would be received within 24 hours that would contain further instructions along with the support ticket number. While there doesn't seem to have been any resolution as of yet, the issue seems to be somewhat of a continuation of when Google was banning Google accounts last year which had belonged to users that were purchasing Project Fi Pixels and then reselling them for profit. While there's no clear indication so far that this is what's happening and the IMEI blacklisting is just a different method being used instead of the account bans, it seems to follow along the same lines. At the very least the issue is a good frame of reference for people to be cautious when buying a Pixel or Pixel XL second-hand.