Some country interconnections, in particular those to UK and Nordic countries, have the ability to connect even unmatched phases, by for example converting to and from DC (AC -> DC -> AC).
That ability to connect unmatched phases is secondary to the primary function of those interconnects: over those distances it is more efficient to transmit the power in DC instead of AC. [0]
Sometimes connecting unsynchronized grids is the primary reason. The Outaouais HVDC Intertie is only about 35km long, but it allows the import and export of electricity between Ontario and Quebec.
Even more fun is that Denmark despite its' very tiny size is split in two when it comes to electricity. West Denmark is connected with East Denmark through HVDC.
This was how the Tres Amigas Superstation was supposed to work.
Unfortunately, ERCOT decided they didn't want to be subject to the same regulations as the Eastern and Western interconnects, so the project collapsed.
