apt-get update
apt-get install asterisk

nano /etc/asterisk/sip.conf

/* sip.conf
[7005]
type=friend
context=muj
secret=123
host=dynamic

[7006]
type=friend
context=muj
secret=456
host=dynamic

nano /etc/asterisk/extensions.conf

/* extensions.conf
[muj]
exten => 7005,1,Dial(SIP/7005,25,r)
exten => 7006,1,Dial(SIP/7006,25,r)

asterisk -r
restart now