How does the Net-Net 4000 ensure that the pinholes are kept open for persistent communication to SIP endpoints behind a NAT device?
By sending OPTIONS messages at a defined frequency
By sending ARP requests at a defined frequency
By changing the expires= parameter to a defined value in 200 OK responses to REGISTERs
By changing the expires= parameter to a defined value in 200 OK responses to INVITEs
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