What is the time delay for the backup 3G Wireless broadband link to become active when the primary link fails?
For a router where the primary link is an ADSL connection and the backup connection is a 3G wireless broadband connection, there is approximately a 5 second delay before the 3G connection becomes active. When the primary ADSL connection is restored, it will take up to one minute for the ADSL connection to become active and the 3G connection to be terminated. This is the time required for the ADSL line to re-sync and PPP authentication to occur.
In either case you should allow a further 40 seconds for SIP registration to occur if you have VOIP configured