[Xiaomi Mi Max ROMs, Kernels, Recoveries, & Other D] E90 Commie:
I had the E90 and 9300i. Excellent devices.

I always cringe when people claim that "Apple invented the smartphone" when the fact of the matter is simple: Nokia launched the smartphone chain (an uninterrupted chain of devices) in 1996 with the 9000 Communicator.

Regarding the power: Switch to airplane mode. If you have two SIMs and the radios are constantly looking for signal it would explain a lot.

That is also why I avoid SIM based call/text services like a plague because it would force me to always have that radio on and it isn't particularly power efficient on any device (HSPA+/LTE use a lot more power than GSM) I have used. WiFi is typically power efficient even in standby mode. The preferred course of action is to have different type of IP based communication and then automatic airplane mode on WiFi connections and then only use the cellular network when being off such a network.

When my device use 0.0-0.1% per hour it is with WiFi connection and airplane mode on (the airplane mode radios are edited so only cellular goes off, WiFi is not affected when switching).

A lot of power saving can be done through automation, i.e. aiming for "app and context based profiles" running services only when needed (such as auto location mode when entering Maps and then off again on exit). The beauty is when there are heavy power savings enabled AND the device is fully automated meaning that it isn't acting like it has been restricted in normal use.

Tasker and AutoInput, AutoTools etc can do a lot there.

It is also possible to use tasks for disabling and enabling services but they require a bit of detailed programming and timing (typically delay the app boot time a bit so the services get enabled first, then starting the app itself in the process).

---------- Post added at 09:39 AM ---------- Previous post was at 09:27 AM ----------

If you are very much into making IP calls (VoLTE is essentially a carrier version of VoIP since LTE networks are data only so regular circuit switch calls are incompatible) I would rather recommend you to check such providers rather than tying yourself to a SIM card.

VoLTE is a sensitive, typically messed up standard where operation sometimes requires proprietary carrier code and testing (I am looking at you, T-Mobile US). What breaks it is sometimes when a Hydrogen ROM is "repacked" for Helium.

One discussion on that particular subject indicated that some "permissions change" when a Hydrogen ROM is repacked into a Helium one and that the VoLTE goes off due to that change. I am still not convinced that the "fault" lies with the ROM rather than the way VoLTE works since the rest isn't affected (i.e. LTE for data etc work).

Both VoLTE and WiFi Calling are pretty pointless features from a technical perspective since they are seriously restricted "VoIP" implementations tied to a SIM card. Why use a VoIP service based on a SIM card when the nature of things are that they can be used independently? If IP calling over WiFi and LTE is important - get a decent provider of VoIP and use it instead.

Every time I see arguments revolving around "carrier X is better because they have VoLTE and WiFi Calling" or "buy device Y because it supports VoLTE and WiFi Calling on [carrier] I smile. I have both services through my Google Voice. Without any SIM restrictions.