SIP Registration is only for telling the Nehos SIP Proxy which IP and port to send a call (inbound DID) to the customers equipment.
The industry default is 3600 seconds, i.e., 1 hour. It should not be set higher than this. It’s also not necessary to set it lower than this or it will just consume more bandwidth and processing power sending unnecessary SIP Registration packets over the internet.
SIP Registration is not needed for outbound calls to Nehos or if using IP Authentication option for SIP Trunks.