Memrise provides a phrase that sounds like da yoyo liang. Is yi becoming yo here, and if so under what circumstances would this change occur? Is it just a dialect issue or standard mandarin.
In characters, the phrase is
Dǎ yāo yāo líng
「一」is not yāo; rather, a different word「幺」is used. See Pronunciation of 壹 / 一 in Mandarin for the use of yāo.
I believe it is this
dǎ yāo yāo líng
(It's the police emergency phone number.)
It is very common practice in mainland China to say "yāo" in place of "yī" when pronouncing phone numbers. I've heard it's because 一 (yī) and 七 (qī) sound so similar.
I don't believe pronouncing yī as yāo is done outside of phone numbers, e.g., 2011年 would be pronounced “èr líng yī yī nián”. See https://chinese4kids.net/year-month-date-in-chinese/
It's not used when you're reading numbers normally, 我想买一百一十一个 is "wǒ xiǎng mǎi yībǎi yīshíyī gè".
When I went to Taiwan, the famous building 台北101 was pronounced by the taxi driver “Táiběi yī líng yī”, so my impression this is a mainland China custom.