Looking for Fixed Mobile Convergence (FMC) Solution

Hi everyone.

We look for a Solution which enables FMC (Fixed Mobile Convergence) for our PBX?

It should work with Android and iOS.

We just looked around but we did not find a Solution which has support for FreePBX or Asterisk.

Our customers want to have only one Number. If the 3G is bad, than the PBX should make the Handover to the GSM Network.

Please help us to find a solution for this.
We want to offer our customers this feature with PBXAct and as well for FreePBX.