Syntax error near "#5018" in android sqlite database

In Android application, I have a database to store contacts. In that, I have a "Number" column of "Text" type.
I get syntax error near: "#5018" (code 1) error while executing below query:

string passedNumber = "#5018"; db.delete("Contact_Table, "Number=" + passedNumber, null);

But If I use quotte aroung number like:
db.delete("Contact_Table, "Number='" + passedNumber + "'", null);
then it is working properly.

My concern is why it is giving error although I have Number column of Text type.