Each Pokemon has a base call rate of either 0, 3, 6, 9, or 15. 0 is reserved for extraneous species that get called in, like Mareanie or Blissey. Of course, there's also no chance of a call if two opponents are still on the field, if the only opponent is inflicted with a status, or (in USUM only) if there's already been one successful call in this chain and you haven't used an Adrenaline Orb yet. Anyway, the "call rate" represents the percentage of time it will call for help provided it's able to do so, at the end of a turn where it's in green health and you have not yet used an Adrenaline Orb. That percentage can only have two possible types of multipliers:
- green health, with Adrenaline Orb: x2
- yellow health, no orb: x3
- red health, no orb: x5
- yellow health, with orb: x6
- red health, with orb: x10
As such, a 15-call-rate Pokemon is guaranteed to call for help every single turn in the 10x case, unless it's prevented from doing so by having a status or having the second slot survive.
Separately from that, there's the chance that the call actually succeeds. By default, the 3/6/9/15 call rate groups have success rates of 12/24/36/60, and being in yellow/red heath doesn't improve those chances at all. The oft-cited Pressure, Intimidate, and Unnerve abilities don't actually affect the call rate, but they do affect the odds of success, giving it a 20% boost to 14.4/28.8/43.2/72% respectively. The only other bonus that can improve this chance is the "momentum bonus":
- opponent called for help last turn but "Its help didn't appear!": x4.5
- opponent called for help last turn, and it was answered, but you promptly OHKO'd it with a super-effective hit: x3
- opponent called for help last turn, and it was answered, but you KO'd it with something that wasn't super effective: x1.5
Using an item, switching up which side of the SOS battle is the "caller" (if you're going for a non-Harvest approach), or just biding time while you wait for the new opponent to do something that might identify what ability they have, all forcefully bring your momentum bonus to a close, and you have to go without that bonus until you get another successful call. 15-rates with the red health, orb, and Pressure bonuses just need 1.5x momentum to make sure that not only do they call for help every turn without fail, but those calls are also answered without fail, so you don't need to worry about lining up type matchups there. But for lower call rates, the x3 is a big benefit to helping the chain go faster, if you have a suitable SE move to mow everything down in rapid succession.