I'll give you an example of why it might not work. Since your phone has roaming, you happen to have it with you at work, or at a party, or at the library, or anywhere really. If even a single acquaintance of yours is "nearby", the information is leaked. If acquaintances seem to always be "nearby", children, wives, husbands, siblings, your info is DEFINITELY leaked.
If anyone is going to try to use this strategy for anything which might result in the loss of your livelihood, (eg - porn), please realize there are many, many, many more precautions you will have to take than are listed in oedfmarap's comment. If you just do what you see in that comment, you could find yourself without a job somewhere down the line.
[1] https://www.defcon.org/images/defcon-22/dc-22-presentations/...
[2] https://splinternews.com/facebook-is-using-your-phones-locat...
I used that foreign number to create my Instagram account and I've gotten the benefit of only being shown suggested accounts from locals from that country (zero people I know). Same goes for ads as well. Currently I keep it on roaming and actually use it to verify other online services that may stubbornly require SMS.
Might be worth a try for those of you looking to pseudo-opt-out of phone number tracking & recommendations on social media services that do this, if you can get your hands on one.