As a someone working in telecom it sounds odd that the OS would be able to influence this, or especially be the prime suspect.
The only fair comparison would be Android vs Sailfish on the same hardware.
Xperia X vs Xperia 10 II can have a couple of dozen other differences in modem firmware and capabilities, e.g. band support.
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I am also experiencing this âbugâ and believe it is due to too long a gap until the phone establishes a new 4G connection after finishing a call (which now is 2G). I hope VoLTE support will fix this. (Xperia 10 II, SFOS 4.4, Deutsche Telekom SIM)
So basically confirming the other reports
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Thank to everyone for all the helpful clarifications and confirmations.
Iâve created a fresh internal bug about this, and will removed the âfixedâ tag while leaving it marked as âtrackedâ.
What would be really helpful would be to test it again after the VoLTE changes have rolled out (no date for this yet). As @Inte points out, itâs reasonable to suppose that this will address the issue, so if it doesnât thatâll be particularly important to know about.
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Just one naive idea that might have been brought up earlier: Is it possible that the phone/sailfish simply checks for 3G networks first and due to the absence of this network it takes these 20+ seconds to an imaginary â3G search timeoutâ until the 4G search is performed?
How does this VoLTE-Theory match to my observation, that this only happens if WiFi is off? ([4.2.0.19][4.1.0.24][Xperia 10II] losing cellular network after call - #23 by fridlmue)
Jolla got ofono logs from me with WiFi on and off in zendesk issue #38217.
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It does not relate to that at all. Having VoLTE would remedy having to switch technologies in the first place - thus not have any issues switching back.
So from my observation:
4G network reconnects immediately if WiFi is on and in the very same spot it looses the network totally after a call for these 10-30 seconds if WiFi is off.
So if all of this issue is connected to the CSFB handling of the phone or the network and not the OS, how could it be that it is working in one of the cases. So I would still suspect that there should be a solution for this bug on the OS scale not requiring us waiting for a final VoLTE Implementation as workaround?! 
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That is an interesting observation, and if it holds true when cross-checked definitely something to look in to.
Perhaps the OS can do more to speed up the process at a failure, even though it shouldnât necessarily need to (or some other similar borderline case like that).
But what i was saying is that iâd be surprised if the OS was somehow stalling or breaking it.
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Issue seems to be fixed in 4.4.0.64! (Xperia 10 II) german provider Drillisch.
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Unfortunately not for me. The same behavior as in all SFOS 4.x.x.x versions on my X10II.
Same with me, same Issue here on the 10 II. Bug still present.
Make sure WiFi was off, when you tried to reproduce
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@fridlmue I switched the WiFi off and tried again. The Issue was there again! So the problem is definitely not solved yet.
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From my experience, and therefore I wrote my note, the problem occurs only if WiFi is off. But I still donât know if that is on my device only. So I would be happy if some other people, on whose devices this error exists, test these three cases and report if the errors occur under these three conditions.
- WiFi off
- WiFi on, no WiFi network connected
- WiFi on, WiFi network connected
I understand your tests this way, that in all three tests mobile data connection is on.
Then I can tell you this:
- the problem occurs
- the problem occurs
- the problem occurs not
and - if wifi and mobile data are off? - - - the problem occurs not
If I understand the network management of SFOS right, connecting to wifi has priority over connecting to mobile data if both is switched on.
The moment a wifi connection is possible, the mobile data connection is disconnected.
In this case the problem doesnât occur.
And in my opinion, the reason for this is the switched off mobile data connection, and not the connected wifi.
In my experience, the problem only occurs when there is a mobile data connection, and disappears as soon as it is disconnected.
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I can confirm this behaviour on my Xperia 10 II - Dual SIM / 4.4.0.64 / Fello Sweden.
Test 1:
- Sailfish phone connected to 4G, WiFi off.
- Make a call to Sailfish phone
- Reject call on Sailfish (phone will show 4G is disconnected, lasting for 10-30 s)
- Immediately call Sailfish phone again
- Call goes to voice mail
Test 2:
- Sailfish phone connected to 4G, WiFi on and connected.
- Make a call to Sailfish phone
- Reject call on Sailfish (phone will show 4G is connected)
- Immediately call Sailfish phone again
- Call goes trough to Sailfish phone
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I have the same issue here. Hope that the last update might solve this but the update only brought additional bugs =(
After upgrading my Xperia 10 II to 4.4.0.68 I was able to activate VoLTE with german t-mobile SIM without any problems.
Although the actual problem is not really solved, enabling VoLTE is a workaround for my Xperia 10 II.
So thanks to the developers for the VoLTE option.
Iâm using the 10ii with O2 Germany and VoLTE only works for incoming but not for outgoing calls.
However, after a 2G call the phone now doesnât loose the network any more and 2G/4G data is now available instantly after hangup.
Iâm having this same problem, and having issues with the device (10ii dual-SIM on 4.4.0.68, T-Mobile US) picking up 4G at all. Sometimes it works, sometimes it doesnât. Sometimes if I reset the telephony stack (either through systemctl or Sailfish Utilities), it may work, but itâs not consistent at all. If I pull the SIM out and install it in an Xperia XZ3 on Android, everything works beautifully, so I know itâs not the network.
Iâve got a history of solving problems like these, so Iâm going to get to work on it this evening, see what I can turn up.