[asterisk-users] There is no tone on an outgoing call

[dima]

Hello, everyone.
I'm having a strange problem with my asterisk. After dialing and before
the other side picks up the phone I should hear the tones (I'm not sure
what are they called: piiiiiiiiiiii---piiiiiiiiiiiiii....) and in almost
all cases that is true. However there is a range of numbers where I'm
having this problem. There is no tones, just silence, until someone
picks up the phone. This does not occur when I call to those numbers
with a mobile or regular PSTN phone.
Has anyone experienced anything similar? Where I should be looking to
correct this?
Thanks in advance

P.S. I'm using asterisk 1.2.18. The Dial command is the same for all
calls: _X.,n,Dial(SIP/${EXTEN}@provider-out,45)