I'm getting "spammed" by barclay's bank to get me to pay off ~$200 balance.

I could easily pay it, but the reason I've left this amount unpaid was to understand a bit about how their calling technology works.

A few months ago, after I forgot to pay one month, I started getting calls from different 18xx numbers. I tried blocking the #s as they came in, but would just start getting the calls from new #s.

The call frequency has increased from about 1/day to now about 12/day. Strangely, I will get these calls as soon as I've unlocked my iphone or otherwise interacted with it. Almost never do I get a call when the phone is locked and not using it.

Even more oddly, I will often be in the midst of dialing another number when they call. I am putting together statistics, but in the meanwhile if there is some app notifying the service that makes these calls, how would you investigate?