I watched this video a while ago that said something similar. https://www.youtube.com/watch?v=ispyUPqqL1c

The decline in birthrates isn't related to growing living standards, as poorer countries also have declining birthrates. Turkey has a lower birthrate than the UK, and Mexico has a lower birthrate than the US. Places like North Africa and South India have seen declines in birthrates comparable to the West.

He makes the argument that declining birthrates are due more to a fall in coupling than a fall in people in relationships choosing to have kids. He brings up that birthrates would actually be increasing if marriage rates remained constant. This means that all the incentives countries push such as subsidized childcare or tax breaks to have kids are putting the cart before the horse, as a growing share of young people don't have a partner to have kids with to begin with.

He then brings up that the fall in coupling a country experiences is roughly correlated to the rate of mobile internet usage in that country. 46% of American teens say they use social media "almost constantly" vs. 24% a decade ago. People would rather use social media than go out and meet others. He points to South Asia as an example, as it's experienced a relatively smaller decline in marriage rates, and mobile internet usage there is lower than in the rest of the world.

I suppose it's yet another way that cell phones are impacting society.