Use isGlobalPhoneNumber() method of PhoneNumberUtils to detect whether a number is valid phone number or not.
Example
System.out.println("....g1..."+PhoneNumberUtils.isGlobalPhoneNumber("+912012185234"));
System.out.println("....g2..."+PhoneNumberUtils.isGlobalPhoneNumber("120121852f4"));
The result of first print statement is true while the result of second is false because the second phone number contains f.