Valid values are:
0-newRequests: all requests are sent to the
primary server first, regardless of the last
used server.
1-DNSTTL: the IP phone retries to send
requests to the primary server after the
timeout equal to the DNSTTL configured for
the server that the IP phone is registered to.
2-registration: the IP phone retries to send
REGISTER requests to the primary server
when registration renewal.
3-duration: the IP phone retries to send
requests to the primary server after the
timeout defined by the
account.X.sip_server.Y.failback_timeout
parameter.