Phone numbers + phone number country + account age + behavior can be used to build a trust score. It might not be bulletproof but it cuts down spam enough for now.

Imagine a messaging app for example, a 1 month old account with a Nigerian phone number cold DMs an account in Australia. The likelihood of this being spam/abuse is extremely high. Vs a 5 year old account that mostly messages mutual contacts cold DMing an account in their own country.

In many countries, phone numbers are a proxy for ID and are difficult to get without having a local ID. The countries which have not secured their phone number system will be less trusted by spam filters.