By Drew FitzGerald

T-Mobile US Inc. users in several states reported problems making and receiving phone calls Monday, a widespread service disruption for the newly enlarged wireless provider.

The network issues began in the afternoon at T-Mobile, which is taking its first steps to integrate its April purchase of Sprint Corp. The transaction left the combined company with more than 100 million customers, including those running atop its infrastructure under other carriers' brands.

T-Mobile technology chief Neville Ray said in a tweet Monday afternoon that the carrier's engineers were "working to resolve a voice and data issue that has been affecting customers around the country," and apologized for the inconvenience.

Representatives of T-Mobile competitors AT&T Inc. and Verizon Communications Inc. reported no apparent problems with their networks, though some customers were unable to reach T-Mobile lines.

T-Mobile executives over the past two years touted their engineers' technical know-how as they sought regulators' approval to take over Sprint, arguing that their smooth integration of MetroPCS was a model for how to join two dissimilar networks.

Combining cellphone networks is a technically complex process that often saddles telecommunications companies with higher costs. Sprint's growth stumbled after it acquired rival Nextel Communications Inc. in 2005.

T-Mobile's nationwide network also supports service from its Metro prepaid brand and customers at resellers such as TracFone, Mint and Consumer Cellular. Social-media users reported more problems with T-Mobile-branded lines than with Sprint or Metro.

Write to Drew FitzGerald at andrew.fitzgerald@wsj.com