The company is to locate its business sector customer support team operations here by the end of the year, it has announced.
And it will be the first to handle both T-Mobile and Orange calls since the companies merged in July.
Richard Moat, deputy chief executive of Everything, Everywhere, owners of the joint new business, said: "Greenock remains a key element of our future operations and strategy.
"We intend to invest further in the facility by locating our business sector customer support team operations there by the year end, making it the first contact centre servicing both Orange and T-Mobile business and corporate customers.
"This is a considerable project for the site, as we bring in Orange processes and systems to the centre, and for our employees to equip them with the necessary new skills to support the Orange brand. This is welcome news for our teams at Greenock, and they are responding well to the challenge."
The investment was welcomed by West of Scotland MSP Stuart McMillan.
He said: "It's reassuring to hear the news T-Mobile is planning to invest further in the facility.
"This is positive news for Inverclyde and everyone who is employed at the centre."
Meanwhile, Mr Moat said the impact of the merger on Greenock was 'minimal'.
He said: "Only a few staff in support team roles are directly affected. It is hoped they will find suitable new opportunities elsewhere in the company."
T-Mobile said it was too early to say how much would be invested in the site, or if jobs will be created.