Like most these days I work in an open plan office. Distractions are par for the course but something that I find particularly hard to deal with and somewhat disrespectful is when colleagues take extended phone calls from their seats in the open office. Spending 5 or 10 minutes on the phone I can understand but I've had people take hour-long calls or start dozens of calls. We have dozens of meeting rooms and phone booths spread across the building and while they're often in high demand, there's always one available.
It would be easier if these were personal calls of course, but people are used to taking those in a phone booth. These are always business calls, but company policy prohibits taking any phone calls in open areas. As with most such policies they're ignored by a handful who end up frustrating the many.
Usually the colleagues that do this work for other teams and I don't know them at all, so how can I ask them tactfully to not take extended phone calls for all to hear? I'm looking for scripts or short phrases to use that get my point across without being unprofessional or dismissive.
Bonus question: how can I actually do so when people are still in their call? I feel that at a certain point the rudeness of inflicting your conversation upon everyone on the floor outweighs the rudeness of interrupting their phone call, but I'm not sure how to word such an interruption well. A short phrase to use that's clear and can't be argued with would be ideal.