Here’s the code for the control, (just to make sure there are no unknown differences):

using System;
using System.Text;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.ComponentModel;
using System.Security.Permissions;

namespace Controls.TextBoxes {
  /// <summary>
  /// Summary description for PhoneControl
  /// </summary>
  [AspNetHostingPermission(SecurityAction.LinkDemand,
  Level = AspNetHostingPermissionLevel.Minimal),
  DefaultProperty("Text"),
  ToolboxData("<{0}:PhoneNumberTextCtrl runat=\"server\"> </{0}:PhoneNumberTextCtrl>")]
  public class PhoneNumberTextCtrl : CompositeControl {
    private TextBox phoneTextBox;
    private RequiredFieldValidator phoneRequired;
    private RegularExpressionValidator phoneRegex;
    private Style phoneStyle;

    #region Properties
    [Bindable(true), Category("Appearance"),
    Description("Phone number for textbox")]
    public string Text {
      get {
        return (string)ViewState["Text"] ?? String.Empty;
      }
      set {
        ViewState["Text"] = value;
      }
    }
    [Bindable(true), Category("Behavior"),
    Description("Is this field Required?")]
    public bool Required {
      get {
        return (bool?)ViewState["Required"] ?? false;
      }
      set {
        ViewState["Required"] = value;
      }
    }
    [Bindable(true), Category("Behavior"),
    Description("Error Message to display if textbox is left blank")]
    public string RequiredErrorMessage {
      get {
        return (string)ViewState["RequiredErrorMessage"] ?? String.Empty;
      }
      set {
        ViewState["RequiredErrorMessage"] = value;
      }
    }
    [Bindable(true), Category("Behavior"),
    Description("Error Message to display if textbox is left blank")]
    public string RequiredErrorText {
      get {
        return (string)ViewState["RequiredErrorText"] ?? String.Empty;
      }
      set {
        ViewState["RequiredErrorText"] = value;
      }
    }
    [Bindable(true), Category("Behavior"),
    Description("Validation Group the control belongs to")]
    public string ValidationGroup {
      get {
        return (string)ViewState["ValidationGroup"] ?? String.Empty;
      }
      set {
        ViewState["ValidationGroup"] = value;
      }
    }

    [Bindable(true), Category("Behavior"),
    Description("Is this field Required?")]
    public bool FormatRequired {
      get {
        return (bool?)ViewState["FormatRequired"] ?? (false);
      }
      set {
        ViewState["FormatRequired"] = value;
      }
    }

    [Bindable(true), Category("Behavior"),
    Description("Text to display if the phone number is not in the correct format")]
    public string FormatErrorMessage {
      get {
        return (string)ViewState["FormatErrorMessage"] ?? String.Empty;
      }
      set {
        ViewState["FormatErrorMessage"] = value;
      }
    }

    [Bindable(true), Category("Behavior"),
    Description("Text to display if the phone number is not in the correct format")]
    public string FormatErrorText {
      get {
        return (string)ViewState["FormatErrorText"] ?? String.Empty;
      }
      set {
        ViewState["FormatErrorText"] = value;
      }
    }
    [Bindable(true), Category("Behavior"),
    Description("Regular expression used to format the phone number")]
    public string FormatExpression {
      get {
        object b = ViewState["FormatExpression"];
        return (String.IsNullOrEmpty((string)b)) ? @"(((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4})|\d{10}|\d{7}" : (string)b;
        //return (string)ViewState["FormatExpression"] ?? String.Empty;
      }
      set {
        ViewState["FormatExpression"] = value;
      }
    }
    [Category("Styles"),
    DesignerSerializationVisibility(DesignerSerializationVisibility.Visible),
    PersistenceMode(PersistenceMode.InnerProperty),
    Description("The strongly typed style for the TextBox child control.")]
    public Style PhoneTextBoxStyle {
      get {
        if(phoneStyle == null) {
          phoneStyle = new Style();
        }
        return phoneStyle;
      }
    }
    #endregion

    public PhoneNumberTextCtrl() {

    }
    protected override void AddParsedSubObject(object o) {
      // Declaratively specified child controls get added through this method.
      // However, the DatePicker builds its own child controls, and does not
      // want to allow the page developer to add controls, which could potentially
      // mess up the control's collection.
      throw new Exception("Cannot add child objects declaratively.");
    }
    protected override void OnPreRender(EventArgs e) {
      RenderJavaScript();
    }
    protected override void CreateChildControls() {
      Controls.Clear();
      phoneTextBox = new TextBox();
      phoneTextBox.Text = Text;
      phoneTextBox.ID = "phoneText";
      phoneTextBox.Attributes.Add("onblur", "formatPhone(this)");
      Controls.Add(phoneTextBox);
      phoneRequired = new RequiredFieldValidator();
      phoneRequired.Enabled = Required;
      phoneRequired.ErrorMessage = RequiredErrorMessage;
      phoneRequired.Text = RequiredErrorText;
      phoneRequired.ValidationGroup = ValidationGroup;
      phoneRequired.ControlToValidate = phoneTextBox.ID;
      Controls.Add(phoneRequired);
      phoneRegex = new RegularExpressionValidator();
      phoneRegex.Enabled = FormatRequired;
      phoneRegex.ErrorMessage = FormatErrorMessage;
      phoneRegex.Text = FormatErrorText;
      phoneRegex.ValidationGroup = ValidationGroup;
      phoneRegex.ControlToValidate = phoneTextBox.ID;
      phoneRegex.ValidationExpression = FormatExpression;
      Controls.Add(phoneRegex);
    }
    protected override void Render(HtmlTextWriter writer) {
      CreateChildControls();
      //EnsureChildControls();
      AddAttributesToRender(writer);
      if(phoneStyle != null) {
        phoneTextBox.ApplyStyle(phoneStyle);
      }
      phoneTextBox.RenderControl(writer);
      phoneRequired.RenderControl(writer);
      phoneRegex.RenderControl(writer);
    }
    private void RenderJavaScript() {
      String csname2 = "formatPhone";
      Type cstype = this.GetType();
      // Get a ClientScriptManager reference from the Page class.
      ClientScriptManager cs = Page.ClientScript;
      // Check to see if the client script is already registered.
      if(!cs.IsClientScriptBlockRegistered(cstype, csname2)) {
        StringBuilder sb = new StringBuilder();
        sb.Append("<script type=text/javascript>");
        sb.Append("function formatPhone(phoneTextBox){\r\n");
        sb.Append("\tvar text = phoneTextBox.value;\r\n");
        sb.Append("\ttext = text.replace(/\\D+/g,\"\");\r\n");
        sb.Append("\tif(text.length == 10){\r\n");
        sb.Append("\t\t phoneTextBox.value = \"(\" + text.substr(0,3) + \") \" + text.substr(3,3) + \"-\" + text.substr(6,4);\r\n");
        sb.Append("\t}\r\n");
        sb.Append("\tif(text.length == 7){\r\n");
        sb.Append("\t\t phoneTextBox.value = text.substr(0,3) + \"-\" + text.substr(3,4);\r\n");
        sb.Append("\t}\r\n");
        sb.Append("}\r\n");
        sb.Append("</script>"); cs.RegisterClientScriptBlock(cstype, csname2, sb.ToString(), false);
      }
    }
  }
}

And here is the relevant content in the ASPX page. Clearly I’ve not made much use at all of the various properties of your control:

<%@ Register Assembly="WebControlLibrary1" Namespace="Controls.TextBoxes" TagPrefix="cc"%>
...
<cc:PhoneNumberTextCtrl ID="ctl" runat="server" Text="some text">
</cc:PhoneNumberTextCtrl>

The text value “some text” is perfectly reflected in both the design-time and runtime rendering. No postback necessary to get expected result. Even when I programmatically set the Text property in the Page_Load() method, the control renders correctly because the call to CreateChildControls() updates the Text property of the child text box by assigning to it the current value of Text stored in view state.

I don’t know why it doesn’t work for you, but I hope the culprit will soon be found.