I'm trying to get a Shelgon and/or Salamence via SOS in Pokemon Sun. no luck, 50+ calls already. Does anyone know how often they appear via SOS?
Serebii have got this information up. It seems that nobody has realised that.
Shelgon has a 10% chance of being found in Kala'e SOS battles. However, Salamence has a 1% chance of being found in SOS battles triggered in specific patches of Route 3. Theoretically you would encounter one in every 100 SOS battles - you've only done 50, so it's reasonable for it to not appear yet. However, I feel you may be going wrong by not running in the specific patches indicated by Serebii in this picture:
Salamence should be found in the patches indicated by the black head thing.
Be patient - it'll take a while!
Hope I helped. :)
Source: Shelgon's and Salamence's respective Serebii pages