@harrysintonen I just meant that the equipment is available in portable form, doesn't have to be centered around an embassy, it can be hidden in a van for example. More technical description here:
https://www.endoacustica.com/passive-mobile-gsm-interceptor-system-wide-band.php

@modrobert Okay sure, that is possible, in theory. Yet in order to get very high coverage of the central Helsinki area you need to overpower large number of cells. Even then you would only get some of the traffic, not full coverage.

The HS article in question claimed that the Russian embassy itself could listen to phone calls in central Helsinki area, which clearly is incorrect (except very next to the embassy).

@harrysintonen Yes, it needs to be close to the target phones and base station, but the system is passive (decrypts and records all calls in parallel) so not sure what you mean regarding "overpower large number of cells".

@modrobert What I mean with it being active is that "passive radio surveillance" traditionally means that you merely need to capture the traffic and not actively transmit. Merely listening to signals is much easier than actual two-way communication. As we know femtocell will actively communicate with the device and thus is not passive in this sense. Some without better understanding can easily misunderstand this, especially if terms like "passive" is thrown around casually.

Overpowering large number of cells meaning that in order to have coverage over area which already has very large number of cells (such as Helsinki central area) you would need to have similar or better coverage with your rogue cells than that which is already existing. This requires very large amount of effort and resources. This with assumption that you would want to listen to phone calls of larger area, such as central Helsinki. More targeted attacks are of course another matter.

@harrysintonen Yes, it seems unrealistic and risky to make a big operation out of something like this, my guess is that this is done on specific targets during limited time. Remember Victoria Nuland's "Fuck EU" phone call that was leaked and became news? Where she decides who will be the next president of Ukraine back in 2014. Anyway, I bet they monitored all her calls at the time.

@modrobert One should also remember that it is fairly easy to mitigate this threat if you know what you're doing. Understanding the threat and following the rules (no classified information over phone calls) goes a long way. For sensitive but not classified information one can use Signal (or similar).

How well people are briefed on these threats and how good their discipline is following the rules varies of course. Good understanding of the threat and adequate briefing of key personnel is essential to countering it.