The call shows as an invalid number even though it's a valid number. What's going on?

• Connection issues - call not connecting between phone providers (just like sometimes one dials a number and it doesn't connect at the time of the call).
• Privacy filters - increasingly, phone service providers and device manufacturers are filtering calls based on databases and screening algorithms, and sometimes legitimate calls get blocked inadvertently.
• Call screening system - the recipient could have recently activated, sometimes without knowing, a call screening or similar service with their phone service or phone device that does not allow our call to go through.