The Opis 60s/PushMeFon mobiles currently (as of 2018) use quad-band 2G network technology. This means that they can operate on any 2G network worldwide. However, in some countries such as the USA, Canada, Singapore, Australia and Taiwan some network operators have discontinued 2G service.
There are currently two further network technologies in operation around the world: 3G and 4G (the latter also known as LTE). We are not contemplating a switch to 3G technology as in many cases network operators tend to view it as a stopgap technology between 2G and 4G (LTE) and in several cases 3G service will be switched off at the same time as 2G service in the countries affected.
We are considering a switch to 4G (LTE) technology. The problem, however, is that while the network is available worldwide there still isn't a global standard (or indeed any stable solution) that allows one LTE device to participate in voice communication everywhere in the world. The voice call technology for 4G (LTE) is called Voice over LTE (or VoLTE for short), and, sadly, it hasn't been standardised and there are no feasible solutions available yet, that would allow global 4G (LTE) technology voice coverage.
In summary, currently our quad-band 2G network still offers the best global coverage, but we are actively following developments in the 4G (LTE) and Voice over LTE field and will switch once this becomes viable. Unfortunately, we expect this not to be the case before 2021 at the very earliest.
Specific Country Information
Valid as of Mar 2018 to the best of our knowlegde (please also see: https://en.wikipedia.org/wiki/2G)
(If in doubt, please ask your network provider if they can provide a SIM-card for 2G. We can assume no responsibility for the correctness of this information.)
Compatible 2G networks include: Telstra (Christmas Islands only)
There are no compatible 2G networks.
All network operators are 2G capable (T-Mobile and Tele 2 - which uses T-Mobile's network are considering a shutdown in 2020)
Compatible 2G networks include: Vodafone
Compatible 2G networks include: TPG Telecom
Compatible networks include: Swisscom (considering a shutdown by 1.1.2021), Sunrise&Salt (shutdown planned for end of 2018)
Compatible 2G networks include: all (NCC may be planning a 2G shutdown sometime in the future)
Compatible with all networks, except iD Mobile.
Compatible 2G networks include: T-Mobile, Cellular Abroad, CTExcel, Jaguar Mobile, KidsConnect, Liberty Wireless, LycaMobile, MetroPCS, Mobal, NTT DoCoMo USA, Simple Mobile, Ultra Mobile, Univision Mobile, UppWireless, US Mobile, UVA Mobile, Value Wireless, Walmart Family Mobile, ZIP SIM, and others.
All other countries
Service should be available via the mainstream network operators. (Some speciality network operators - notably 3/Three - may surcharge for use of a 2G service or block it. It is best either to avoid them or to find out the exact conditions beforehand.)