UNITED STATESSECURITIES AND EXCHANGE COMMISSIONWASHINGTON, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

CSG SYSTEMS INTERNATIONAL, INC.

(Exact name of Registrant as Specified in Its Charter)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 1.01 Entry into a Material Definitive Agreement.

Summary of Material Definitive Agreement. CSG Systems International, Inc. (“CSG”) is a party to a Master Subscriber Management System Agreement, dated August 1, 2017 (the “Current Agreement”) with Charter Communications Operating, LLC (“Charter”). The Current Agreement is scheduled to expire on March 31, 2028. For the second quarter ended June 30, 2025, CSG generated approximately 19% of its total revenue from Charter.

On September 4, 2025, CSG entered into an amendment (the “Amendment”) to the Current Agreement. The key terms of the Amendment are:

• The Amendment extends CSG’s contractual relationship with Charter through September 30, 2031.
• The revenue to be generated under the Amendment will be based primarily on monthly charges for SaaS and related services per Charter customer account, and various other ancillary services based on actual usage. The Amendment includes reduced price escalators for 2025 and annual fixed-price escalators beginning in 2026. CSG did not provide a renewal discount to Charter in the Amendment.
• The Amendment contains certain financial commitments both throughout the term and associated with the number of Charter customer accounts that are to be processed on CSG’s solutions.
• CSG maintains the exclusive right to provide print and mail services to all current and future Charter customer accounts through the term of the Amendment.
• The Amendment contains certain rights and obligations of both parties, including the following key items: (i) the termination of the agreement under certain conditions; (ii) various service level commitments; and (iii) remedies and limitations on liabilities associated with specified breaches of contractual obligations.

A copy of the Amendment, with confidential information redacted in accordance with SEC rules, will be filed as an exhibit to CSG’s Quarterly Report Form 10-Q for the quarter ended September 30, 2025.

Item 7.01 Regulation FD Disclosure.

The following information, including Exhibit 99.1 hereto, is furnished pursuant to Item 7.01 (Regulation FD Disclosure). This information shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or incorporated in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as shall be expressly set forth by specific reference in such a filing.

On September 4, 2025, CSG issued a press release announcing that it had entered into an Amendment with Charter. A copy of the press release is attached to this Form 8-K as Exhibit 99.1 and is incorporated into Item 7.01 by reference.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

EX-99.1

EXHIBIT 99.1

CSG Announces Contract Extension with Charter Communications

Expansion of long-standing relationship creates significant value for both companies

DENVER, Sept. 4, 2025 – CSG® (NASDAQ: CSGS) and Charter Communications today announce that they have signed a multi-year contract extension and expansion of their decades-long relationship. With this extension, CSG will continue as a revenue management and monetization partner for Charter through September 30, 2031, and will expand its support to include new areas of growth in Charter’s business.

“For more than 25 years, CSG has supported Charter teams in delivering innovative communications, entertainment and connectivity products and services,” said Mike Woods, EVP and President of North America Communications, Media and Technology, CSG. “We are honored to serve this industry leader into the next decade.”

As part of this expanded relationship, Charter will leverage CSG’s industry-leading, cloud-native SaaS platform, CSG Ascendon, for its seamless entertainment experience, which combines live TV with access to programmers’ streaming apps. Coupled with the CSG ACP billing platform, Ascendon will deliver the flexibility needed to support today’s digital entertainment services. Charter will continue to rely on CSG’s platforms to support its residential Internet, TV and home phone customer base.

Combining revenue and customer experience management capabilities with AI-powered analytics, CSG Ascendon is the ideal solution to power new digital offerings for leading global brands.

Learn more about how CSG Ascendon can help accelerate growth by unlocking new revenue streams, speeding up time to market and engaging customers at critical moments.

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About CSG

CSG empowers companies to build unforgettable experiences, making it easier for people and businesses to connect with, use and pay for the services they value most. Our customer experience, billing and payments solutions help companies of any size make money and make a difference. With our SaaS solutions, company leaders can take control of their future and tap into guidance along the way from our fiercely committed and forward-thinking CSGers around the world.

Want to be future-ready and a change-maker like the global brands that trust CSG? Visit csgi.com to learn more.

Contacts:

Julia Dakhlia

Public Relations

+1 (402) 431-7376

julia.dakhlia@csgi.com

John Rea

Investor Relations

+1 (210) 687-4409

john.rea@csgi.com