you said you had put surge protection on the interface port but you did not mention whether you have UPS on the PBX itself. They could have less than stable power at the site which might not affect other cards in the system. The only other place this ight show up is in the PMS or office computers but could almost guarantee that those units have UPSs on them. Just a thought. And sorry for missing the Fidelio in original post (I'm Polish what the heck do you want) and yes they are pretty reliable.