Zoiper just would NOT ring on the Huawei Y360 if I left it alone for a while. I tried SIP UDP, SIP TCP and IAX and this cheap $60 phone which I actually bought just to have a cheap phone to use for phone-calls (it's dual-sim) and VOIP. I was close to testing how well it would fly if I threw it out the window.
It appears that Mediatec, in their infinite wisdom, has decided to give this phone a "deep idle state". This state can be configured in /sys/power/dpidle_state
It is possible to make it not go into this very annoying "deep idle state" by giving it this command:
echo cpupdn 0 > /sys/power/dpidle_state
This phone does react to incoming VOIP phone-calls when "deep idle state" is turned off.
I (ab)used King Root too root this phone after installing all available updates (don't root first and try to update later because bad things will happen). I strongly recommend against using King Root because it places all sorts of nastyness in a variety of folders and if you don't know how to replace it with SuperSU and remove it's garbage then you're stuck with it (I don't know any other easy way of rooting it, though). If you do have SuperSU on the phone then you can put the above command in /system/etc/install-recovery-2.sh which SuperSU runs after it installs it's daemon using /system/etc/install-recovery.sh
Not allowing the phone to go into the Deep Idle State likely drains more battery. This is a simple trade-off, these cheap Huawei phones apparently don't run anything in the background (I tried things like Wifi Keep Alive etc) out of the box. That obviously saves a lot of battery but a longer lasting battery may not be worth it at the cost of not being able to use the phone to run background apps.
Perhaps this will help someone, perhaps not. I just felt like share that if your cheap Huawei phone doesn't take incoming phone calls then this is probably why.
Just for reference, some other things I tried:
* Settings -> Protected apps, [x] Zoiper so it doesn't get closed in low memory situation. I kept this setting even though it didn't let me have incoming phone-calls.
* Added com.zoiper.android.app to /system/etc/resmonwhitelist.txt since rumors on the internets claimed this would keep it alive. I also left that there even though this also didn't let me have incoming phone-calls.
* Zoipers Config, Connectivity should all be checked (Keep Wifi Alive, Stay Awake, Run in Background) though none of these things are actually enforced on this phone if you let it go into deep sleep.