Arthritis Research UK suggest that there is no evidence that collagen supplements have any effect whatsoever on arthritic joint pain. There is some (very small) evidence that they might have a preventative effect, that is to say that they may be helpful in preventing the onset of deterioration of tendons and cartilage. This research is still very contentious.
Yes, this is true that collagen supplements can help joint pain relief. Some people I know have much more improved mobility, and little to know pain from take a natural collagen supplements. 1 guy avoided hip replacement surgery, and another I know had a very bad shoulder where he could not raise it above his head, and needed surgery. Both after taking the supplements had major improvement in mobility and actual joint pain relief.
]Some collagen supplements have additional ingredients like glucosamine and bladderwrack. These 2 components are similar in helping reduce joint pain or inflammation of the joints by binding to inflammation creating lectins, which cause inflammation / pain. Additionally they have other healing properties too, but work great with collagen in reducing inflammation and providing collagen for regrowth to damaged tissue / area causing the pain.