Here’s one way I just figured out to make this happen; possibly not the only or best way, but seems to do the job. Relevant config sections follow. Idea here is two SIP clients (one PC and one Android device) with two google voice accounts, and enough configuration for both devices to place and receive calls on both lines.
*** jabber.conf
[general]
debug=yes
autoprune=no
autoregister=yes
[XXXXjabber]
type=client
serverhost=talk.google.com
username=XXXX@gmail.com/Asterisk
secret=supersekret
port=5222
usetls=yes
usesasl=yes
statusmessage="Asterisk"
timeout=100
[YYYYjabber]
type=client
serverhost=talk.google.com
username=YYYY@gmail.com/Asterisk
secret=supersekret
port=5222
usetls=yes
usesasl=yes
statusmessage="Asterisk"
timeout=100
*** sip.conf
[androidXXXX]
type=friend
host=dynamic
secret=sekret
context=gvXXXX
callerid=“AndroidXXXX” <101>
nat=yes
[androidYYYY]
type=friend
host=dynamic
secret=sekret
context=gvYYYY
callerid=“Android YYYY” <101>
nat=yes
[pcXXXX]
type=friend
host=dynamic
secret=sekret
context=gvXXXX
callerid=“pcXXXX” <100>
nat=yes
[pcYYYY]
type=friend
host=dynamic
secret=sekret
context=gvYYYY
callerid=“pcYYYY” <100>
nat=yes
*** gtalk.conf
[general]
allowguests=yes
bindaddr=x.x.x.x
[guest]
disallow=all
allow=ulaw
context=gv-inbound
connection=XXXXjabber
*** extensions.conf
[users]
exten => 100,1,Dial(SIP/pcXXXX,20)
exten => 101,1,Dial(SIP/androidXXXX,20)
exten => 200,1,Dial(SIP/pcYYYY,20)
exten => 201,1,Dial(SIP/androidYYYY,20)
[gvXXXX]
;append an area code if necessary
exten => _NXXXXXX,1,Set(CALLERID(dnid)=1239${CALLERID(dnid)})
exten => _NXXXXXX,n,Goto(1239${EXTEN},1)
;append a 1 if necessary
exten => _NXXNXXXXXX,1,Set(CALLERID(dnid)=1${CALLERID(dnid)})
exten => _NXXNXXXXXX,n,Goto(1${EXTEN},1)
;do our real dialing
exten => _1NXXNXXXXXX,1,Dial(gtalk/XXXXjabber/${EXTEN}@voice.google.com)
exten => _+1NXXNXXXXXX,1,Dial(gtalk/XXXXjabber/${EXTEN}@voice.google.com)
include => users
[gvYYYY]
exten => _NXXXXXX,1,Set(CALLERID(dnid)=1239${CALLERID(dnid)})
exten => _NXXXXXX,n,Goto(1239${EXTEN},1)
exten => _NXXNXXXXXX,1,Set(CALLERID(dnid)=1${CALLERID(dnid)})
exten => _NXXNXXXXXX,n,Goto(1${EXTEN},1)
exten => _1NXXNXXXXXX,1,Dial(gtalk/YYYYjabber/${EXTEN}@voice.google.com)
exten => _+1NXXNXXXXXX,1,Dial(gtalk/YYYYjabber/${EXTEN}@voice.google.com)
include => users
[gv-inbound]
exten => XXXX@gmail.com,1,Answer()
exten => XXXX@gmail.com,n,Wait(1)
exten => XXXX@gmail.com,n,SendDTMF(1)
exten => XXXX@gmail.com,n,Dial(SIP/androidXXXX&SIP/pcXXXX,20)
exten => YYYY@gmail.com,1,Answer()
exten => YYYY@gmail.com,n,Wait(1)
exten => YYYY@gmail.com,n,SendDTMF(1)
exten => YYYY@gmail.com,n,Dial(SIP/androidYYYY&SIP/pcYYYY,20)