Asterisk

[general]

; Treat 403 responses to registrations as if they were
; 401 responses and continue retrying according to normal
; retry rules.
register_retry_403=yes

; which context (in extension.conf is used as default)
context=default

; A register line is not needed if you connect as a static host.
; This is an option in the advanced VoIP Trunk settings.
register => accountid:password@sipproxy.voipgrid.nl/accountid

[accountid]
type=friend ; User + Peer
secret=password

; Peer settings
host=sipproxy.voipgrid.nl
username=accountid; authentication username
fromuser=accountid; authentication username
fromdomain=voipgrid.nl

 ; Codec settings (use only alaw)
disallow=all
allow=alaw

; Other settings
insecure=invite
nat=never (if you experience one way audio, or use a nat setup, set nat to yes)
sendrpid=yes
trustrpid=yes

[default]
autofallthrough=yes

include => internal
include => external
include => dialrules

[internal]
; Internal numbering plan
[external]
; Numbers that come from "outside" (your DIDs)

[dialrules]
exten => _[0-9+]!,1,Dial(SIP/${EXTEN}@accountid,60,t)

registerattempts=0 ; asterisk will continue registering infinitely after internet loss and won't stop after a certain number of attempts.
registertimeout=5 ; every 20 seconds

maxexpirey = 1800 ; if the trunk regularly loses its registration. You can also try 600 / 300 if needed.
defaultexpirey = 600 ; same

dtmfmode=rfc2833 ; optional if experiencing problems with dtmf tones

externip=myip.nl ; fill in your own external ip address
localnet=10.0.0.0/255.0.0.0 ; fill in local network + mask

asterisk -r
reload