Japan’s largest wireless carrier, NTT DoCoMo, saw its second-quarter profits slump while handset sales and rising data revenue failed to offset continued declines in voice.

The carrier made $1.51 billion, a 20 percent drop from last year as voice revenue fell 5.5 percent. In addition, NTT DoCoMo sold just 4.46 million handsets, 850,000 fewer than last year.

The company also cut its devices sales forecast to 18.2 million units from its prior expectation of 19.7 million. NTT DoCoMo sold 20.13 million handsets last year.

NTT DoCoMo is working to boost its data business with new services and devices. The carrier has launched seven new handsets, including Japan’s first Android phone, and has boosted its mobile video offerings.

In addition, the carrier has ramped up its customer service offerings with free basic handset repairs, simplified mobile payments and changes to its battery service.