I have Problem calling to PSTN

I’m using asterisk-1.4.21.1 and zaptel-1.4.11
The problem is rare because I call to pstn and the call progress but something don’t call to pstn, it’s like if the telephone is up.
In the CLI report the Answred status but i don’t listen the ring tone of the pstn ringing.
I think that the problem is not in me zapata.conf ,zaptel.conf or me extensions.conf because the calls progess sometimes and sometimes not.
I was testing and if I push the numbers slow the call progress
If I push the numbers fast, do not progress. Why?

you need to provide more information, please include the output of a call that goes wrong, and your zapata.conf and include the context for outgoing calls