The N510 will send after a restart of the device a "message-waiting" Subscribe and is using the same Call-ID/From-tag for this subscribe. It can be that the Cseq value is lower then the value before restart and some platforms then to not accept the subscribe, only after the Subscription is expired.
This is how it is designed and the reason is that if you always send a new Subscription, the platform still has the old subscriptions (up to 6 as N510 supports 6 SIP accounts) and after more restarts, you could have many subscriptions open that are not valid anymore.
But some platforms require that they would like that we send a new subscription and not the same.
For this we implemented a new parameter that can only be changed via provisioning.