this is the setting I used for the italian VoIP provider Vira.it

Add a SIP Trunk:

Maximum channels:
empty (potentially a lot but not tested how many)

Outgoing Dial Rules:
empty (I use only vira at moment so I dont need to set anything)

Trunk Name:
vira (or whatever)

PEER Details:
type=friend
username=<**USERID**>
fromuser=<**USERID**>
secret=<**PASSWORD**>
host=sip.vira.it
context=from-pstn
insecure=very
disallow=all
allow=ilbc
allow=gsm
allow=G729
allow=ulaw
allow=alaw

User Context:
(leave this blank)

User Details:
(leave this blank)

Register String:
<**USERID**>:<**PASSWORD**>@sip.vira.it

Next add a DID Route:

DID Number:
<**YOUR VOIP NUMBER**>

Set Destination:
Use 'Incoming Calls' settings