Persist item value if...? (Fritzbox calls)


i want to build a kind of call monitor for my fritzbox. for that i set up the persistence for the incoming call item:
fboxIncomingCallResolved: strategy = everyChange, restoreOnStartup

because i choose everychange as strategy, even the end of the calls are logged as “null##null” in the database. is there a way to except this change from persistence?

Thanks, Alex