I had two cockatiels, both males, that talked but just one or two lines. One said "Baby" and "Pretty Baby", his name. He taught Noel to say both those lines. But if you do not keep up with having them repeat what they know, they forget it and never say it again.
In the wild, cockatiels have a natural call that sounds like a human done wolf whistle, but instead of the last note going lower in pitch and longer, the last note is high and short; so it is easy for the cockatiels to learn the human wolf whistle and it is usually the first thing they master. The key to teaching a youngster to speak is to pick one or two words you want them to say and repeat them to them constantly in two or three minute time periods, as often as you have access to the bird. Cockatiels and budgies like words with 'b' and 'p' in them, so teaching "pretty bird" seems to be easiest for them to master. I changed Baby's name to Benny when I got him and he did learn to say Benny after a few weeks and he would even say "Baby Benny".
One of the reasons cockatiels forget how to talk is that every year, the cocks slightly change their song in response to what the other cocks around them are saying and what other birds live in their area. My cockatiels always pick up cardinal song since I have two sets of breeding cardinals in my yard. Recently a catbird started raising young and they picked up the catbird calls as well. When I brought Benny into my flock and he said 'pretty bird' many of the other cocks tried to learn to say it, but usually just garbled it into indistinct 'b' and 'p' sounds; and Benny picked up the cardinal song and everything else my original flock had as well. Having four or five cockatiels competing in the mating months and singing over one another is a real treat for me, but my BFF finds it annoying noise. I love birdsong, any type of birdsong.