…is to clear your mind and think of a short random word.
Here goes – mmm – ‘dove’
Now pick a number below 100.
Now pick another word.
Here goes – mmm – ‘thunder’
Now pick another number below 100.

Put it together: ‘dove23thunder44’

Sounds a bit like a football score, but it’s pretty unique, unguessable, memorable, and easy to say over the phone. You won’t get attached to it like you do with your dog’s name, so you won’t mind picking a new one every year/quarter.