Bingowings said:

I'm wondering where the call to the other Doctors came from (the Tardis perhaps but she is very rarely understood by any Doctor).

. . . . . What was the need for the Zygons sub-plot other than to pad the episode out (or pay for the suits to be used in later Zygon stories or to justify the new slightly different Zygon action figure) ? . . . . . etc........

I was thinking about this....perhaps 3 doctors failed to save the planet.....and soon after Smith's Doctor regenerates the Capaldi Doctor enlists all previous incarnations to lend a hand.

The Zygon bit was for comic relief & to mirror .....blow up London to save the World/blow up Galifrey to save the Universe