it would really depend on the level of redundancy you want to have and the money you're willing to spend.
if you see a real possibility of your worst case scenario happening sometime then SRST would be a good option.
if you feel that at any time only one of the two scenarios you mention could happen then you might be covered
just take in consideration that depending on the amount of phones it could cause some WAN congestion if not properly sized
HTH
javalenc
if this helps, please rate
HTH
java
if this helps, please rate