Pro: the second line can be deactivated (“disconnected”) from the phone. Con: added cost.

Depends on your local laws and such, but in most European countries you can get a prepaid SIM card for a couple of euros/pounds/whatever at any supermarket, making them practically free. If you need a temporary number for a scammy special offer or any situation where your number is publicly visible (Gumtree, etc) it's a no-brainer IMHO.

If your phone suppprts running two SIMs at once, it has two IMEIs so as far as the network(s) are concerned it's two distinct handsets unless they deduce otherwise.

A fun aside: years ago I did some work for a small phone company (the company was small, not the phone) and they gave me a SIM with 100 numbers in a block and access to a portal I could manage them with. Sadly, I forgot to pay the annual £10 renewal fee.

You don’t have to get a mobile number. You can get a number from a voip provider. Those numbers can accept sms usually. Usually cheaper than mobile number and you can access from your phone with a SIP client.

I got a second number for a while and saw no real benefits and went back to one

Getting another number helps a lot in my opinion. If you're getting another sim in the same phone, I suppose it would be possible for whatever cell carrier is providing the service to be able to link your two sim cards to your identity. However, for other companies, I don't know of a way they could gather that your second sim phone number is linked to your first one. For instance, if you created two Discord accounts with your two phone numbers, you could theoretically be two completely different users that won't be linked together. I would personally go for a second number.

I have been doing precisely that. You have your personal number and the secondary number that doesn't have to be attached to your name if you want. Ofc law enforcement and telecom companies will be able to trace you based on cell tower locations and phone model.

Pro: An extra disposable number to give out whenever people ask you for "we need your phone number for ShittyDiscount". Whenever the spam calls get annoying I simply pop it out and place another prepaid sim.

Con: Phones with dual sim support can be rarer to find and be more expensive depending on your country.

During covid, i made a Google voice number so i didn't have to give my real number when we were working from home. It was fine for that.

This is a good question. Phone numbers are increasingly used as de-facto ID numbers, everywhere in the world. That's because, unlike email, they cost money, and in most jurisdictions you can't even get one anymore without presenting real ID. So: if you have a second phone number, you can effectively have a second persona for any site or app that requires phone-number ID. Seriously, at this rate, it's going to be all of them.

IMO the best use-case for this is to quarantine your contact list. That is, keep a separate number for social networks and messaging. The number you give to your in-person contacts will be instantly shared with all their cloud services, whether you like it or not. This is what allows Big Tech to triangulate and discover exactly who you know and therefore who you are. If the cloud services cannot trace a number back to any phone ID in their own books, then they can't do much with it and you will remain at least something of a mystery to them.

ESIM using silent.link = a phone line that can only be used to receive messages and calls no outbound calls allowed. However you need a phone with esim support.

Physical sim on another device: great but you need a seperate hardware and have something extra to carry and charge.

Physical sim on a dual sim phone. Easier to carry however it runs the battery faster and sometimes you can forget which sim you are on if you are quickly calling or texting.

Mains sim plus VoIP line example is mysudo- use one phone, seperate your communication between your actual number and an app with VoIP. Seperate phones # via software since VoIP is all on app side. Requires you have an internet connection to work properly. Not all services are happy using VoIP and stop you from registering or changing numbers to VoIP.

Lots to think about, best of luck.

It really helps my confused brain to hear all your ideas and suggestions! For me it seems there are more pros than cons, so I will go to the local store and buy a prepaid Sim 🙂