We do this easily with the Angani Cloud PABX
What's your existing IP-PBX equipment in use ? Maybe the IP PBX can even do a SIP Trunk between the 2 IP-PBXes (since this is what IP-PBXes are supposed to be able to do).
But if you want easy mobility like the Android client etc - then your best option is just sign up for the Angani Cloud PABX - this is why we built it - full mobility, no need to purchase equipment, and always-on access to your phone service in a secure reliable datacenter.
Let me know offline if you need further details.
Riyaz