Hello,
If you only want to bridge calls, we are working on a bit of a hack that lets you use a pair of old Android phone (or potentially an android phone and a mesh potato) to gate the audio between mesh and cellular networks via acoustic coupling of headsets, or for an extra couple of dollars electrically coupled via a custom double-ended headset lead.
See recent progress at:
We freely admit that it is not what you would call an "ideal" solution, but a number of people have been asking for it, and it does offer the potential to fairly cheaply and easily connect mesh and cellular networks, without the need for expensive cellular modems or any custom hardware.
Paul.