Thank you Danis for your support. 



The problem was the missing TAP/TUN support in my kernel! Now everything is working.


The sim800 is similar to sim900, it has the same features. I've made this little fix to sim900.c:




420c420
<     if (data->modem_type == SIM900) {
---
>     if ((data->modem_type == SIM900) || (data->modem_type == SIM800)) {



to enable sms and gprs for the SIM800.


Regards


Da: "Denis Kenzior" denkenz(a)gmail.com
A: "Riccardo Sala" riccardo.sala(a)springtech.it, ofono(a)ofono.org
Cc: 
Data: Thu, 16 May 2019 10:39:59 -0500
Oggetto: Re: Simcom SIM800 gprs data connection not working


Hi Riccardo,
 
> 
> # dbus-send --system --print-reply --dest=org.ofono /sim900_0/context1 
> org.ofono.ConnectionContext.SetProperty string:"Active" 
> variant:boolean:true
> Error org.ofono.Error.NotImplemented: Implementation not provided
 
This basically means that no gprs_context atom has been created by the 
modem driver or the driver initialization has failed.  Briefly looking 
at plugins/sim900.c, I can see that only sim900 (not sim800) supports 
gprs.  Given that you get this far, I assume you hacked this part to 
create gprs/gprs_context...
 
The other place to look is to see whether the 'atmodem' gprs_context 
driver is working properly.  Perhaps you're missing the TUN/TAP support 
in your kernel?  Run 'sudo modprobe tun' and see if that fixes things.
 
See drivers/atmodem/gprs-context.c for more details.
 
Regards,
-Denis