Sockets are busy / circuits are busy

Hi,

  I am very new to Asterisk FreePBX. sometimes i make a call to outside i got a message "all sockets are busy" or "all circuits are busy". i m using asterisk 2.2 fonality home version and my voip provider is Inphonex. 
   I installed codecs [b]G.729[/b] and i updated [b]sip.conf[/b] file. my sip.conf file is like this..

port = 5060 ; Port to bind to (SIP is 5060)
bindaddr = 0.0.0.0 ; Address to bind to (all addresses on machine)
disallow=all
allow=ulaw
allow=g723
allow=g729
context = from-sip-external ; Send unknown SIP callers to this context
callerid = Unknown
nat=yes
defaultexpiry=120

Can any one help me about this issue?.

Thanks & regards in advanceā€¦

Sadiq.