[General Questions and Answers] serafis:
I would love someone to help with this strange problem, before I throw this phone in the bin!

First the specs.

Doogee x5Pro, Android 5.1,

Baseband DOOGEE-X5pro.2015/10/08

Kernel version 3.4.5DOOGEEinfo@doogee.cc.2015.10.08

Build number DOOGEE X5pro-Android5.1-R08-2015.10.26

The phone works fine when I set Cellular Settings / Preferred network type to 2G. Of course it only works as a 2G phone in this mode.

However, if I set the Preferred network type to either 3G or 4G (recommended) then as soon as I try to make a call or send an SMS, the modem radio turns off and the call is immediately dropped or Messaging says the SMS service is unavailable. Occasionally the called phone will ring, but the call drops immediately. Sometimes the SMS does get through, but at my end it appears that the modem radio turns off and on again as soon as the Dialler or Messaging tries to use it. If someone tries to call me, the phone rings and caller-ID is displayed, but as soon as I answer, the modem radio cycles off and on again, and the call gets dropped.

If I am in an 4G LTE coverage area, then 4G data works fine (browsing was fast, Speedtest was showing 85+Mbps) but I still cannot make or receive calls, or send SMS's - as soon as I try, the modem radio cycles off and then on. If I am not in an LTE coverage area (but am in a 3G WCDMA coverage area), then the phone just constantly cycles showing 4G for an instant then 3G, then radio off, then radio on but no service, then 4G etc... - this cycle is very regular and takes about 53 seconds end-to-end. If wi-fi is on, this cycle only starts if the modem is accessed (e.g. by trying to make a call), or by turning wifi off and cellular data on. And of course I can't make or receive calls or send SMS's during this cycle, and even when the signal shows 3G I can't get any data connectivity. While it's doing this, if I go to Settings / SIM cards, the 'On/Off' slider moves to 'Off' when the radio cycles off, and then moves back to 'On' again.

In summary; in 3G or 4G preferred network mode and not in LTE coverage, whenever anything (calls, SMS or data) tries to use the modem it cycles off and on again. When in LTE coverage, data works but calls and SMS cause the modem to cycle. As I mentioned, 2G is fine.

I think I've tried most of the obvious things, and some not-so-obvious ones. These are the things I've tried so far.

Obvious things like reboot, battery out, reset to factory settings, wipe cache, clear application data. No change.

I swapped the SIM from Slot 1 (LTE/WCDMA) to Slot 2 (GSM) - the behaviour was exactly the same (this surprised me as it seemed to be LTE-related).

I put my SIM in a carrier's (Vodafone NZ) Samsung 4G phone and it worked fine, also it was fine in a Chinese-brand 3G phone. Then I put another Vodafone NZ 4G SIM in my phone and it had exactly the same problems, so I think it's the phone rather than the SIM or the carrier.

Because data works fine when in LTE coverage, and the phone does ring if called (but cycles the modem immediately when I answer the call), it seems the hardware is possibly working OK. So I assumed software or settings might be the cause.

I created new APN's using carrier-supplied settings. No change. I flashed various Doogee stock ROM versions (don't think there are any custom ones for this phone) and they all display the same problem, rooted or otherwise. I've used the engineering menus (*#*#3646633#*#* and *#*#4636#*#*) to change bands, network preferences and various other settings with no effect. 2G (GSM only) is the only one that works. I installed the Xposed Framework and Gravitybox module and had a play with some of the settings in there but again, no effect.

I don't know what else to try and I can't return the phone as it costs too much to send it back to China from New Zealand.

So any help anyone can give me would be greatly appreciated, otherwise I'm stuck with a not particularly useful 2G wi-fi phone.

Thanks in advance.