Edited Issue: MaskedEditExtender loses value [27920]

MaskedEditExtender works fine when user enters a value then exits the textbox.
However, I have a situation where a textbox is filled by code:
PhoneTextbox.Text = "6105551234" (value found in database, but is user editable)

When the textbox is displayed it is blank! Am I missing something?

Here is the Textbox definition:
<asp:TextBox ID="PhoneTextBox" runat="server" MaxLength="10" Columns="13" Visible="False"> </asp:TextBox>
<cc1:MaskedEditExtender ID="PhoneMaskedEdit" runat="server" Enabled="True" Mask="(999) 999-9999"
MaskType="Number" TargetControlID="PhoneTextBox" CultureAMPMPlaceholder=""
CultureCurrencySymbolPlaceholder="" CultureDateFormat="" CultureDatePlaceholder=""
CultureDecimalPlaceholder="" CultureThousandsPlaceholder="" CultureTimePlaceholder=""
ClearTextOnInvalid="false" >

Please help.

Thank you.