Here is an example of how to get a Vonage SoftPhone number to work with the Asterisk PBX. I am using this at my home. All incoming calls ring both SIP phones. All voicemail goes to the same mailbox.

SIP.CONF

[general]
srvlookup=yes
disallow=all
allow=ulaw
canreinvite=no

; 12125551234 is your Vonage SoftPhone number.
; zzzzzzzz is your Vonage SoftPhone password.
; The number after the “/” is supposed to be an extension configured
; in the [vonage-exten] context. I have found that Vonage always sends
; the Vonage SoftPhone number regardless of the number specified here,
; so I have specified the Vonage SoftPhone number after the “/” so that
; it is clear what extension will be searched for in [vonage-exten].
register => 12125551234:zzzzzzzz@sphone.vopr.vonage.net:5061/12125551234

[vonage-sip]
type=friend
context=vonage-exten
username=12125551234
host=sphone.vopr.vonage.net
fromuser=12125551234
fromdomain=sphone.vopr.vonage.net
secret=zzzzzzzz
insecure=very

; Local SIP phone
[1011]
type=friend
context=secure
username=1011
secret=xxxxx
host=dynamic

; Local SIP phone
[1012]
type=friend
context=secure
username=1012
secret=xxxxx
host=dynamic

EXTENSIONS.CONF

[general]
static=yes
writeprotect=yes
autofallthrough=yes
clearglobalvars=yes
priorityjumping=no

; Secure context for outgoing calls from local SIP phones
[secure]
; Internal calls
include => default
; Parked calls
include => parkedcalls
; Emergency calls
exten => 911,1,Dial(SIP/${EXTEN}@vonage-sip,60,r)
; Local calls
exten => _NXXXXXX,1,Dial(SIP/${EXTEN}@vonage-sip,60,r)
; Long distance calls
exten => _1NXXNXXXXXX,1,Dial(SIP/${EXTEN}@vonage-sip,60,r)

[vonage-exten]
; An incoming call from Vonage rings local extension 1011 and 1012
; All voicemail goes to mailbox 1000
exten => 12125551234,1,Dial(SIP/1011&SIP/1012,20)
exten => 12125551234,n,Voicemail(su1000)
exten => 12125551234,n,Hangup

; Press * during voicemail greeting to retrieve your messages
exten => a,1,VoicemailMain(1000)
exten => a,n,Hangup

[default]
; Local extension to retrieve your messages
exten => 1000,1,VoicemailMain(s1000)
exten => 1000,n,Hangup

; Local SIP phone at extension 1011
; All voicemail goes to mailbox 1000
exten => 1011,1,Dial(SIP/1011,45)
exten => 1011,n,Voicemail(su1000)
exten => 1011,n,Hangup

; Local SIP phone at extension 1012
; All voicemail goes to mailbox 1000
exten => 1012,1,Dial(SIP/1012,45)
exten => 1012,n,Voicemail(su1000)
exten => 1012,n,Hangup