Doesn't solve the problem if they agree to pay X.

@BittermanAndy Choose X such that it's high enough that you're willing to deal with them for that much.

@BittermanAndy: smithsonianmag.com/history/… (Making chalk mark on generator $1. Knowing where to make mark $9,999.)

I disagree. While this may be a good idea, I don't think it's THE most effective way to get them to stop. The most effective way is to ignore them.

@only_pro - No, the most effective way would be to scream obscenities down the phone at them.

@only_pro The objective of this answer isn't necessarily to make them stop. It's to make them realize that they're taking up OP's time and that time comes at a cost. Straight up ignoring them will annoy them and possibly burn bridges, which may or may not be desirable. Charging a consulting fee to answer their questions forces them to choose to either not bother OP with questions or pay OP's consulting fee, both of which are favourable outcomes.

Getting cash in pocket might take the sting out of hearing from them too.

I agree with this answer as it makes your feeling clear without burning bridges unnecessarily. A nice round figure like $500/hr for work with a flat $200 fee per phone call should do the trick

Once you stop working for a company, you... stop... working... for... that... company. You don't keep working for them, whether paid or unpaid. If it was about money, the former employer could have paid the OP enough to stay, and they would have stayed. That didn't happen. Therefore this does not answer the question, which was, "how do I make them stop and leave me alone", not "how do I take on more work for myself while making sure I get paid for it".

This might depend on the new employer being happy for you to take calls for your new consultancy side-gig while you're at work, of course.

@DavidAldridge If that is a factor, one could direct the old employer to ask the current employer's sales department for a quote.

@Haem True, so it would depend on the new company being willing to move into a new area of consultancy, taking on this client, and the OP would not see any financial benefit from the arrangement?

@LVDV: It's certainly wrong for the Netherlands. You can't charge VAT, though, but that's not a problem. The UK also has Independent Contractors, but they indeed need to register with HMRC.

OK guys, back to reality. I get how all y'all want to totally feel awesome by "owning" that old company, but are you even aware that most companies have that "no work for others" clause in their contracts? Which kinda turns this into advice to commit a crime...

@R.Schmitz That's not a common clause nowadays and a friend who works in the netherlands got that scrapped from his contract in a heartbeat when he was presented with it once. Additionally in a lot of cases those clauses aren't even enforcable in the Netherlands and the most common enforcable kind is that you require written permission which can't really be denied unless it clearly influences the work the employee does (e.g. if you pay an employee to handle emergencies outside work hours when they arise).

@DavidMulder Well I heard that about these clauses where you can't work in the same sector after you left the company for X years, that that isn't enforcable. But working for another company while you are employed at this company? That doesn't even seem unreasonable to me (as it happens I'm also in the Netherlands and do have such a clause in there).

The one who bills is should of course be the current employer. As you are an employee of another company, it might actually be illegal to help any other company.

@TeroLahtinen being an employee of a company doesn't make that company own your life. If you are not doing it while on the company's time and don't use their IP, I don't see how it would be illegal.

Glad to see the practical difficulties in the comments. When OP chooses to go this route, they will either have to start a business and deal with the extra administration that comes with it, or they need to approach their employer in order to have the billing go through them. If the employer does not have a consulting service yet, I don't think they'd be very willing to set one up. Especially when the goal is to stop the calls, since OP doesn't have the time for this extra responsibility. This low effort answer sounds very cool, but in practice Joe Strazzere's answer is the most effective.