Verizon and Japanese carrier KDDI signed an agreement this week that will offer Voice-over-LTE (VoLTE) roaming capabilities to KDDI customers visiting the United States.

The deal, which Verizon said is the first from a U.S. carrier to offer VoLTE services to foreign travelers, will go into effect this month.

Though the agreement does not yet include VoLTE roaming capabilities for Verizon users visiting Japan, KDDI president Takashi Tanaka said the carrier is looking “forward to providing Verizon’s subscribers the same experience in Japan in the very near future.” Verizon said it is currently testing VoLTE roaming with “multiple carriers” and expects to offer VoLTE and HD voice roaming in Japan later this year.

The deal marks the latest progression of VoLTE services from U.S. carriers.

Back in November 2014, Verizon teamed up with AT&T to work on VoLTE interoperability. A month later, AT&T followed up with an announcement it was working on VoLTE interoperability with “other” wireless carriers.

Earlier this year, T-Mobile said “well over” half of calls made on its network are carried via VoLTE.

Though AT&T has previously touted “reliability advantage,” a recent report from Amdocs  called that assertion into question. According to Amdocs data, the number of dropped calls on VoLTE can be four to five times higher than calls on a 2G or 3G network.

In terms of network outages, though, VoLTE beats LTE. A March report from Spirent Communications said 80 percent of network operators surveyed reported VoLTE outage or degradation issues occur either rarely or not at all.

Where VoLTE degradations or outages do occur, operators pointed to VoLTE client performance in the device and mobile core performance issues as the top two causes.