Configure your Asterisk PBX server through IP authentication. This type of authentication is used when you have static ip address and this is default scheme.
You need to give us your static ip address.

sip.conf:
[sip_proxy]
type=peer
host=87.238.224.117
disallow=all
allow=g729
nat=no
context=ringring
usereqphone=yes
outboundproxy=87.238.224.117

Then you may use this configuration in your dialplan anyway you want, for example to send all calls in the context you may do the following:

extensions.conf:

[ringring]
exten => _X., 1, Ringing()
exten => _X., 2, Wait(1)
exten => _X., 3, dial(SIP/${EXTEN}@sip_proxy)