Posting this here, copied and pasted from my post to Reddit. Might be good info for people who use this phone on Straight Talk with AT&T service. Don't know if it applies to their other carriers:
UPDATE: ONE BUG WITH MMS: I'm using this phone on Straight Talk with an AT&T SIM. There seems to be something going on with MMS messages that is specific to Lollipop on this phone: You can't send or receive them using most Messaging apps. This applies to Google Messenger, the stock Messaging app, and a few others I tried. The only app that seems to send/receive MMS picture messages properly is Hangouts.
I came to this conclusion after trying the same APN settings (which the phone defaulted to), some older APN settings that had worked (att.mvno, MMS PROXY:
http://mmsc.cingular.com", MMS Proxy: 66.209.11.33, MMS Port 80, etc), and a lot of experimentation mixing/match/looking for others on the internet.
Using the att.mvno settings OR the default settings the phone downloads (APN: tfdata, etc), Hangouts is the only app that will send or receive group texts and picture messages.
I can't find a work-around. I'm not thrilled at having to use Hangouts for texting, but at least it functions.