The account is said to be worth more than £100,000. Text is aiming to raise Amdocs' corporate profile in the business and tech arena via a media relations drive.
In July, Amdocs bought Bath-based Cramer Systems, which provides support systems for telecoms firms including BT, for £205m.
The St Louis firm uses Access Communications in New York, Porter Novelli in Germany and Shalmor Scherf Strategic Communications in Israel.
Amdocs recently hired The Jeffrey Group, Text 100's partner agency in Brazil.