Things are moving quick in the mobile world. Interoperability is an issue in the Voice over LTE world, as it has always been in cellular communication. Currently, the standards that were pretty constant in the “old world” of GSM, have been drifting apart fairly quick, and the TMF has been asked to push “the toothpaste back into the tube” by making standards globally accepted, while many carriers rather enter into bilateral agreements than spend time on standardization issues.
Nevertheless, TMF have been fairly good at doing precisely that. Yet, here we go and see another bilateral agreement implemented, read more here: