Reliable predictable steady communication is probably an unrealistic expectation of someone with BPD once things slip out of the early performance mode. If you can't accept that, and you stay engaged, there will be a lot of discomfort for you. Most people, very reasonably, want predictable reliable communication from their partner.
For whatever reason, that part didn't bother me all that much. What did bother me was when I came to grasp that the non-communication was due to him spending time with other women. And since he was masterful at obscuring that from me ("sorry, my phone was in my bag" I had to work my way via spidey sense to the realization that the break in communication meant something more.
You have to decide for yourself, not with reference to other people and their choices and what they don't mind and what they can't tolerate, what YOU can't tolerate; what you need, from someone you share intimately with.
I would caution against making assumptions about WHY he is not answering. It probably is not that he doesn't care. He may be dysregulated. He may be immersed in something. He may be immersed in something because he is dysregulated ... .
. Whatever the reason, it's probably not realistic to think it will change in any sustained way, and so you need to assess whether you can deal with that.
