Hair transplant scar, strong dermal filler needed (Photo)

I prefer nano fat grafting (also known as micronized fat) for scar therapy.  This invloves the removal of fat from the hip, filtration to a very thin substance, then injection with a fine needle directly into the dermis to introduce stem cells for remodeling.

Permanent fillers may work but you would need to find a doctor who have done these on hair transplant scars.  Even with the fillers, you will still notice the scar to some degree.

You can consider using bellafill or even sculptra to fill your scar, these are still semi-permanent fillers. You could also consider doing a scar revision or something along the lines of scalp micropigmentation. 

Hair transplants should NEVER be taken from the neck as they form scars that are different than from the scalp. Fillers are the only way to relieve the depression and scalp micropigmentation will make the scar non-visible. See below

If you've had Voluma injections and they've helped improve your scar temporarily, then I agree- you probably could get longer duration improvement with a more permanent class of filler, such as Bellafill. A Bellafill kit contains 5 one cc syringes, and you would likely  use all 5 of them for a scalp scar as the one you describe. Roughly 20% of each syringe is PMMA, the rest is essentially collagen. This means that within about 2 months the collagen is gone, but the permanent PMMA remains. This is the "permanent" part that can stimulate intrinsic collagen production, giving you long term results. Yes, there are other products that might provide benefit (Radiesse or Sculptra), but your experience with these would probably be similar to Voluma. Good luck.