Heads up as I see people going back and forth about working vs not working VM and then VVM. VM (standard voice mail) works fine as can be. VVM (visual voice mail) does not. Ultimate test of this (as other have said this is associated with the OS not handling VVM on dual sim Win Phone models both current and past). I will hopefully in next couple days have second sim populated with service (both from ATT as it turns out). So.. Both from ATT both with VVM service capability on the accounts but no doubt VVM will not work on either (even if it was a conflict of SIM 1 vs SIM 2 not having same settings for VVM, this would solve that as they would both be using the same APNs for everything and same voice mail).
Side note, I went from ATT 920 to unlocked 950 XL. Didn't use old SIM, had our company rep (inside company and ATT rep at ATT) provision a new SIM. Placed sim in the phone, turned it on fort he first time, and had LTE instantly and perfectly work standard voice mail, texting and hotspot functionality. Old 920 with no active SIM was tested as a client to hotspot functionality of the 950 XL, I could browse faster in Edge browser on the 920 connected via hotspot then I ever could natively with working LTE (same plan, connecting to same towers as I tested this at same physical location right after we had the SIM for the 950 XL provisioned).
Note worthy yet, as others noted before, LTE/4G will change out during calls/etc. But no performance impact noticed while it is happening or before/after it happens. Also forced the phone to sync with service provider for APN settings (says it updated, no change in 4G/LTE/etc). Will see what days bring on the network if their system sweeps through and changes anything.