How much of the decrease in national crime is due to people choosing just to play on their phone instead of do crime?

None, and I can offer proof:

• The Taking of Pelham 123 (1974) the crime was committed without cell phones, but in the 2009 remake with Denzel Washington they had cell phones and still chose to commit the crime.
• The Manchurian Candidate (1962) we have a political assassination without cell phones. But in the 2004 remake with Denzel Washington the cell phones do not stop the criminals from being criminals.
• Man on Fire (1987) the crime is a kidnapping. Then in the 2004 remake (with Denzel) again we see the same kidnapping is still committed with cell phones.

You add cell phones and Denzel Washington to the situation, and seems like nothing much changes.

None.

Crime is primarily driven by poverty and other inequities when social supports are lost or dismantled. For the most part people turn to crime when they don't have other options available.

At least once. I was going to rob a bank the other day but decided to play on my phone.

https://www.forbes.com/sites/alexknapp/2013/01/03/how-lead-caused-americas-violent-crime-epidemic/

lead in gasoline caused lead poisoning, leading to an increase in violent crime. now most fuel is unleaded, reducing the incidence of lead poisoning, and leading directly to a decrease in violent crime

You think people do crime because they're bored? It usually because they need to survive or provide for their families in a capitalist dystopia, because they are discriminated or because they lack appropriate physical mental health supports. Often, the three a the same time.

Somewhat related, there's a notable decrease in gang violence un Mexico whenever a new Dragon Ball movie gets released

A lot less than the reduction in brain damaged individuals thanks to unleaded gasoline and lead remediation in homes and schools.

Male sedation hypothesis might have a part in it as well.

The "Male Sedation Hypothesis" proposes that digital stimuli like video games, pornography, and social media are sedating young men, reducing their tendencies for status-seeking and reproductive behaviors, which historically might have led to instability in societies. theorizes that these digital interactions provide men with titrated doses of what would traditionally drive their social and reproductive pursuits. For example, men receive reproductive cues from pornography and camaraderie or status in video games, leading to reduced real-world interaction and aggression. This is potentially creating a less volatile but more socially detached male population. Williamson perceives this as a mixed blessing; while it prevents disruptive behaviors, it may render these young men less effective in real-world situations that demand assertiveness or direct action

So the gist of it is basically that in prior times like this, the discredited young men have taken it to the streets, assembling in groups of other similar men and start causing trouble by breaking windows and kicking grannies. However, we're not seeing this now despite the record numbers of such men and one hypothesis for why this is, is that those men are instead withdrawing from the society into their mom's basements and spending their time playing games, watching porn and smoking weed.

Religion doom scrolling is truly the opiate of the masses.

Play GTA on my phone rather than in the street? Hell ya!

No, it is based more on the weather.