In a series of very minor updates, three of Verizon's DROID line will be getting Advanced Calling 1.0 enhancements. The Ultra, Maxx, and Mini will each receive what amounts to the same update, which is described as no more than "bug fixes and stability improvements" to Verizon's VoLTE calling service. For the unfamiliar, Advanced Calling promises far clearer voice quality by using only LTE when possible and on supported phones.
The changelog above also applies to the DROID Maxx. The Mini update announcement comes in a separate document, but has the same brief changelog. If you haven't yet received the update, don't worry, as it should come in a matter of days. Either way, this is not a big change that you need to get too worked up about when it doesn't show up immediately.
Sources: Verizon (PDF, Ultra and Maxx) (PDF, Mini)