First you got:
Gateway of last resort is 10.1.2.2 to network 0.0.0.0,
and then you got:
R* 0.0.0.0/0 [120/1] via 10.1.5.5, 00:00:00, Serial0/1
Which route will be used to go to a network that has no route shown in the routing table?
It seems here are two possibilities, takes 10.1.2.2 or 10.1.5.5?