The first comic was released in May 2014, and the second towards the end of 2014 just after Christmas. Since then they have been released with less than a year between them, so one comic has been released in every year from 2015 to 2018. The years 2014 and 2017 are the only years to have seen the release of two xkcd phones as of November 2018. In that way it probably follows well in the footstep of the real smartphone producing companies.
Another comic simply called Phone (1802: Phone) was also released in 2017, but apart from the title and that it is about smartphones there is no relation.