In case this helps:

I had the same thing happen to me. SL2100, just installed. POTS lines. I had done the line calibration already, and it had a fresh restart. Came back a couple days later to see how things were going, and they tell me ext 101 had 99 messages from two phone numbers. Every message was 4 minutes apart with no audio or actual message. 99 is the max number of messages as set in 47-02.

This happened overnight, with auto attendant answering. One of the calls was me, checking out the DAT choices and routing, and I did not leave a message.

I cleared out all messages and its been ok since.

Jim