External sip extensions on Android devices

We have a Asterisk with some internal (on the same lan) extensions and some external ones (on the Internet).
The external extensions are on Iphones and Android devices.
The ones on Android devices are not stable and looses connection to the pbx, Asterisk shows Unreachable for the specific extensions when it happens. iphones devices are stable and do not suffer from these issues.
We tried to change the keep alive and reregister value, but same issues remains.
We tried Bria, Zoiper, and a few others, with the same results.

Anyone can help with an idea ? Is there something special on Android that can lead to such issues ?

Marius KaizermanITAsked:
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     You have a networking issue but there is not overview of the connectivity or carriers involved. here.  You basically said I have a box, how big is it ?
      Are you logging anything in var/log/asterisk/full or var/log/messages.  
      Have you considered using something like OpenVPN to get you connection up and then connecting to Asterisk -

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