I was standing in a long line, and watched two people at the counter trying to simultaneously mail parcels and talk on their phone. In both cases the postal worker had to explain things multiple times, and wait for them to finish chatting for a second before paying.
Fortunately, I'm not friends with any such people, but I wonder if the phone conversation can be any less distracted than the in-person one. The person on the other end must know their friend is at the post office, and hear half of the conversation with the employee. I presume this person is multi-tasking both conversations equally poorly.
If I were on the phone with someone, and they said "Hold on a moment, I'm next in line at the post office", I would not be offended.