Filed pursuant to Rule 424(b)(7)
File No. 333-117427

PROSPECTUS SUPPLEMENT NO. 1
(TO PROSPECTUS DATED NOVEMBER 2, 2005)

$230,000,000

2.50% Senior Subordinated Convertible Contingent Debt
SecuritiesSM (CODESSM) due 2024
and
Common Stock Issuable Upon Conversion of the CODES

     This prospectus supplement relates to the resale by the holders listed below of the 2.50% Senior Subordinated Convertible Contingent Debt SecuritiesSM (CODESSM) due 2024 of CSG Systems International, Inc. and the common stock issuable upon the conversion of the CODES. You should read this prospectus supplement together with the prospectus dated November 2, 2005, which is to be delivered with this prospectus supplement.

     Investing in these securities involves certain risks. See “Risk Factors” beginning on page 3 of the prospectus.

     Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved these securities, or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is February 6, 2006





SELLING SECURITYHOLDERS

     The table below sets forth additional or different information concerning beneficial ownership of the CODES and the common stock, and supplements and, to the extent inconsistent with, amends the table appearing under the caption “Selling Securityholders” beginning on page 56 of the prospectus. To the extent that a selling securityholder is listed both in the table below and in the table appearing in the prospectus, the information set forth below regarding that selling securityholder supersedes the information set forth in the prospectus. We have prepared this table based on information given to us by the selling securityholders listed below prior to the date hereof. Unless otherwise set forth in this prospectus supplement, footnote references in the table below are to the applicable footnote set forth in the prospectus. In addition, the information relating to KBC Convertible MAC28 Fund, KBC Convertible Opportunities Fund, KBC Multi Strategy Arbitrage Fund, Melody IAM Fund and The Drake Offshore Master Fund, Ltd. set forth in the table on pages 58, 59 and 61 of the prospectus is hereby deleted.

Name of Selling Securityholder   Aggregate
Principal
Amount of
CODES
Beneficially
Owned That
May Be Sold		   Percentage
of CODES
Outstanding
Before
Offering		   Number of
Shares of
Common
Stock Held
Before
Offering (1)		   Number of
Shares of
Common Stock
That May Be
Sold (2)		   Number of
Shares of
Common
Stock Held
After
Offering (3)		   Percentage
of Common
Stock
Outstanding
After
Offering (3)		  
 
 
 
 
 
 
 
                             
KBC Convertible Opportunities                            
   Fund, a segregated portfolio of                            
   KBC Alpha Master Fund                            
   spc(5)(24)   $ 2,500,000   1.09%   93,388   93,388   —   —  
KBC Convertibles Mac28                            
   Limited(5)(24)   $ 200,000   *   7,471   7,471   —   —  
KBC Multi-Strategy Arbitrage                            
   Fund, a segregated portfolio of                            
   KBC Alpha Master Fund                            
   spc(5)(24)   $ 2,150,000   *   80,313   80,313   —   —  
Melody IAM Limited(5)(24)   $ 150,000   *   5,603   5,603   —   —  
Sub Total   $ 206,250,000   89.67%   8,391,124   7,704,510   686,614   1.39%  
All other holders of debt                            
   securities or future transferees,                            
   pledgees, donees, assignees or                            
   successors of any of those                            
   holders   $ 23,750,000   10.33%   N/A   887,186   N/A   N/A  

*      Represents less than 1.0%
 
(24) Carlo Georg, Chief Investment Officer of KBC Alternative Investment Management Limited, the investment manager for each of KBC Convertible Opportunities Fund, KBC Convertibles Mac28 Limited, KBC Multi-Strategy Arbitrage Fund and Melody IAM Limited, on behalf of these selling securityholders, has voting and investment power over the securities identified above as beneficially owned by these selling securityholders.
 

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