Quality issue with VoIP calls to cell phones

Why are VoIP calls to cell phones ALWAYS low quality? I have noticed that all our calls to cell phones have consistently high jitter, lag and distortion even though the cell phone has good reception and if I call the cell from a land line the quality is excellent. Vice cersa if I call VoIP to landline the quality is also excellent. Only VoIP to cell is bad.

We use Asterisk 1.6xxx with PIAF (FreePBX 2.8) and g729 codec. I am assuming it’s a transcoding and/or feedback issue between the 2 systems but don’t know if there is a way to fix this.

Does anyone know any settings in Asterisk that would address this issue? Thanks in advance.