[Serusers] Registrar / Expires & reject frequent refreshes

Hello all, I have an issue with some SIP clients (like Linksys SPA 1001) that have not comply with the "expires" parameter of the Contact header of a "200 OK" Registrar Response. My SER conf : modparam("registrar", "default_expires", 1800) modparam("registrar", "min_expires", 300) modparam("registrar", "max_expires", 1800) Although the (200 OK) response contains the 'expires' parameter : Contact: <sip:userb2@192.168.2.10:5061>;expires=300;received="sip:1.2.3.4:5061" the SIP client ignores it and keeps sending REGISTERs every 5 sec (if configured so). Is there a way for SER to reject these registrations with (423 Interval Too Brief), according to RFC3261 - 21.4.17 (423 Interval Too Brief) & 10.3 (Processing REGISTER Requests). The 'retry_after()' registrar modparam is not my case. thank you, Kostas