Apparent inconsistency in Console Log reporting of modem activity

Question

I have a MAC Pro using (in this case) 10.4.11. It has an internal "on hold modem."
When I connect to the internet, the Console Log consistently and reliably adds this item:
"Dec 23 00:13:42 jpnoaks-7500 /System/Library/Extensions/IOSerialFamily.kext/Contents/PlugIns/InternalModemSu pport.kext/Contents/Resources/AppleModemOnHold.app/Contents/MacOS/AppleModemOnHo ld: MOHServiceAddedProc >>\n"
HOWEVER, if I'm already connected and there's an incoming call, SOMETIMES an identical item is added in the Console Log and SOMETIMES nothing is added. The "on hold" features of the modem otherwise appear to behave normally.
Is there a ready explanation for this inconsistent behavior? It happens even when successive incoming calls are from the same phone and when an incoming call is from a phone that has never before called (although I'm not entirely certain about the latter).
Obviously, I'm trying to track modem activity using the Console Log.
TIA

Answer 1

I'm still not absolutely certain about this, but it would appear that there's no Console Log entry added if the incoming call is from a phone
number that called in recently. How recently? -- I don't know.
One way I can see that this would be normal behavior would be if the computer keeps a record of incoming phone numbers and doesn't
report any event involving that number in the Console Log until the record is refreshed, which could be every hour, day, or whatever.
Is it possible the computer is keeping a record of incoming phone numbers?