The company explictly says that being on-call is not mandatory, there is no problem about this. Other colleagues at my same level are doing on-call and bring this topic (among other topics) to the salary review process to be considered in their favour.
I'm not going to change my decision, but I'm wondering how that will influence on my future salary. Even if the companay is transparent about this topic, I definitely think that not participating in "non-mandatory" stuff in my company makes me no favour.
At equal aptitude, employees with a higher engagement with the company are more valuable.
They have finite resources, and obviously those resources will be allocated in a way that benefits the company the most.
So yeah, you should expect your coworkers to get a bigger raise, and you should be fine with it. They won't love you all equally, it's a company.
If the company has an official policy saying that on call is not mandatory, that is good. That said, there is all kinds of corporate doublespeak and unspoken assumptions with these kinds of things, like with unlimited vacation. Even if they are sincere, know that the consequences may be more than a lack of raises. You may lose opportunities for advancement, be put on less interesting projects, or given more mundane roles.
I personally am like you, and don't want to either work on-call or be promoted into management anyway. Many people are like that, especially as they get older and have kids. Most non-startup companies have a healthy need for some SWEs to be on-call, but certainly not all or even most of them. However, there are certainly some workplace cultures with that startup mentality that equate working at all hours and being on-call with being a good employee. Those are the kinds of places that celebrate when everyone needs to work ridiculous hours nights and weekends to ship before a deadline, rather than seeing that kind of crunch time as an unfortunate failure of management that should not happen again. If you're in one of those companies, my only advice is to start looking elsewhere.
If you want to continue to get raises and especially advance, you'll have to prove yourself in other ways, especially if it sounds like you're also opting out of other "non-mandatory" social activities. Be exceptionally competent, reliable, and responsive during your work hours. Work well with others, both on the surface and deep in the work itself. Smile, laugh, and make smalltalk with the people who like smalltalk but not with the people who don't. Don't call unnecessary meetings. Also, write excellent and well-organized documentation about your products and workflows, so that if something goes wrong during the nights or weekends, you're not going to be a blocker.
You can say no. They can give you a smaller raise for it. Honestly, I don't see this as a problem - you get to choose between maximizing raises vs. setting boundaries in your work. It is great that they give you that choice, as most places I know would enforce being on-call as part of your base salary.
Proofing systems against faults, drilling for specific responses, and looping in SWEs (such as yourself) to take on support obligations for their own stack, architeccture, build, and deploy decisions are also strongly recommended faault-proofing and response-expediting measures.
This also incentivizes the company not to mess around with on call for dumb problems. A user is locked out at 3am? Not a $400 problem. The server goes down at 3am? That is a $400 problem.
If you really want to stick to your guns with time over money, then make the on-call bonus a time-in-lieu situation. For every hour worked on-call you get half a day of time off.
Of course, you can always just stick to your guns. "Part of the reason I chose this job is that I don't have to do on-call duties. It is a company policy that it is voluntary. Changing that policy materially changes the terms of my employment in a way that matters very much to me." That last sentence is written in a way that will make HR people nervous without being impolite.
Any manager, or HR person can read right between the lines of that, which is essentially: "Try me. Best case is a renegotiation of my contract. Worst case is that I jump ship to any of the unlimited competitors that will give me what I want."
Depending on circumstances, you might never be assigned call at all, while still checking the box on your performance review.
And/or
Since you value time more than pay, demand a multiplier, but paid in comp time.
So that instead of working more hours, you end up working fewer.
Joanna : Really? I... I have fifteen pieces on. I, also...
Stan, Chotchkie's Manager : Well, okay. Fifteen is the minimum, okay?
Joanna : Okay.
Stan, Chotchkie's Manager : Now, you know it's up to you whether or not you want to just do the bare minimum. Or... well, like Brian, for example, has thirty seven pieces of flair, okay. And a terrific smile.
Joanna : Okay. So you... you want me to wear more?
Stan, Chotchkie's Manager : Look. Joanna.
Joanna : Yeah.
Stan, Chotchkie's Manager : People can get a cheeseburger anywhere, okay? They come to Chotchkie's for the atmosphere and the attitude. Okay? That's what the flair's about. It's about fun.
Joanna : Yeah. Okay. So more then, yeah?
Stan, Chotchkie's Manager : Look, we want you to express yourself, okay? Now if you feel that the bare minimum is enough, then okay. But some people choose to wear more and we encourage that, okay? You do want to express yourself, don't you?
Joanna : Yeah, yeah.
Stan, Chotchkie's Manager : Okay. Great. Great. That's all I ask.
I venture to say that, if they paid you a billion dollars per hour, you would bend your own rules a little. In a case like that, I would do nothing else but be on-call for a few months, then live a life of luxurious leisure for the rest of my life.
The thing then is to price your time so that it is out-of-reach for the company. If you can't manage to do that, you have two options: (A) knuckle-under and do the on-call. (B) leave the company.
It's not only the time/money but first-hand exposure to system issues.
If you have to be called at 3am (and it's happened to me more than once), the workday is completed when the problem is fixed.
A smart employer does not want you to come in during the normal workday and doing something stupid, or get in an accident driving.