1. 30 seconds would give you enough time for a bit of back and forth, while still being refreshingly brief.
1.1 Or, make it just under a minute, as you'd otherwise be throwing away the fraction of the minute not used.
2. Block the caller from calling again for x minutes, set by the receiver.
I'd love to be able to get a hold of call connection data and see what the average cost-per-minute of a call is. I'm sure there's a lot of calls where it runs 1 min 10 sec. or so, and that 50 seconds is wasted but still charged (as it's per-minute).
It would be interesting to try to get the partner provider to value a 'SmallTalk' call around what the actual profit for a call is (vs. published rates), for a little more transparency (and cost control) to the pay as you go market it would make sense in.