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The PRACK Timer is used to request an extension of the transaction at proxies in case the INVITE transaction takes some time to generate a final response (RFC 3262 page 3). When the PRACK Refresh Timer expires the IMG 2020 will send out a 1xx reliable response.
60 (seconds) - Wait 60 seconds before sending the 1xx message.
150 (seconds) (default) - Wait 150 seconds before sending the 1xx message.
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