Okay, that makes sense. The manual has changed for newer versions.

The lockout condition is caused by excessive data errors on the keyset(s). It can be caused by a defective phone, cable issues or the keyset card itself.

How many keyset ports are locked out? If it’s more than one, I’d start by swapping with a known good keyset card and test. If the phones don’t come up, start looking at cabling issues.