We will check the various BB's to see what we can see. We ourselves have only one customer who uses analog (4 sites), but they don't use CLIP and we've never had a failure of this nature with them (their oldest site is just coming up for its second birthday).
For us, analog isn't worth the hassel so we only ever install BRI and PRI (mostly PRI). Digium Analog is very Ma Bell orientated and it shows in the zaptel code, which is bodged up to hell and back for BT and other networks.
Sorry we can't help more but this really does look like an Asterisk or Digium card issue.
Best
S