I assume you get this error when you validate a empty number.
Comparing an empty value with < or = operators will return an error. Since calculations with empty/null isn't technically possible.
You can solve this by adding them empty check also in the second decision like this
$Contact/PhoneNumber != empty
and
$Contact/PhoneNumber <= 9999999999 and $Contact/PhoneNumber >= 1000000000
Another option is to move the second decision between the first decision and the first merge in the true flow.