Jabarkan Interoperability and global roaming dari Universal Mobile Telecommunications System dalam bahasa sunda

At the air interface level, UMTS itself is incompatible with GSM. UMTS phones and UMTS data cards sold in Europe, the United States, much of Asia, and South Africa, are UMTS/GSM dual-mode devices, hence they are backwards compatible with regular GSM networks. If a UMTS customer travels to an aréa without UMTS coverage, a UMTS phone will automatically switch to GSM (roaming charges may apply). If the customer travels outside of UMTS coverage during a call, the call will be transparently handed off to available GSM coverage.

Regular GSM phones cannot be used on the UMTS networks.

Softbank (formerly Vodafone Japan, formerly J-Phone) operates a 3G network based on UMTS compatible W-CDMA technology, that launched in December 2002. Lack of investment in the network through 2003 méant that coverage was slow to expand and subscriber numbers remained low. The network was publicly relaunched in October 2004 and again in 2005, and Vodafone now claims network coverage of 99% of the population, while 15% of their subscribers are 3G users.

NTT DoCoMo's 3G network, FOMA, was the first commercial network using W-CDMA since 2002. The first W-CDMA version used by NTT DoCoMo was incompatible with the UMTS standard at the radio level, however USIM cards used by FOMA phones are compatible with GSM phones, so that USIM card based roaming is possible from Japan to GSM aréas without any problem. Today the NTT DoCoMo network — as well as all the W-CDMA networks in the world — use the standard version of UMTS, allowing potential global roaming. Whether and under which conditions roaming can actually be used by subscribers depends on the commercial agreements between operators.

All UMTS/GSM dual-mode phones should accept existing GSM SIM cards. Sometimes, you are allowed to roam on UMTS networks using GSM SIM cards from the same provider.

In the United States, UMTS is currently offered by AT&T Mobility on 850 MHz and 1900 MHz, due to the limitations of the spectrum available to them at the time they launched UMTS service. T-Mobile will be rolling out UMTS on the 2100/1700 MHz AWS frequencies, but has been delayed by a lack of spectrum availability.  Because of the frequencies used, éarly modéls of UMTS phones designated for the US will likely not be operable overséas and vice versa; other standards, notably GSM, have faced similar problems, an issue déalt with by the adoption of multi-band phones. Most UMTS licensees seem to consider ubiquitous, transparent global roaming an important issue.