Two terminals / one number / SIP forking?

I just setup two “groups” and configure asterisk with my SIP provider.
It is half working : a call to my main phone number rings both phones, and I can pick up but then I have no sound in any direction.
Here is what my config files look like :

extensions.conf

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

[work]
include => parkedcalls

exten => _6XXX,1,Dial(SIP/${EXTEN},20,tT)
exten => _6XXX,2,VoiceMail(${EXTEN}@work)

;voicemail number
exten => 600,1,VoiceMailMain(${CALLERID(num)}@work,s)

;call any number to OVH
exten=> _[*#0-9]!,1,Dial(SIP/to-ovh/${EXTEN})

;Call groups
;When 9000 numers 6001 and 6002 both ring for 40 sec, first one to pick up get the call, otherwise then directed message box
exten => 9000,1,Dial(SIP/6001&SIP/6002,40,tT)
exten => 9000,2,Voicemail(6001@work)

[from-ovh]
exten => s,1,Goto(work,9000,1)

users.conf

[general]
hasvoicemail = yes
hassip = yes
hasiax = yes
callwaiting = yes
threewaycalling = yes
callwaitingcallerid = yes
transfer = yes
canpark = yes
cancallforward = yes
callreturn = yes
callgroup = 1
pickupgroup = 1
nat = yes

[template](!)
type=friend
host=dynamic
dtmfmode=rfc2833
disallow=all
allow=ulaw
context = work

[6001](template)
fullname = kro computer
username = kroc
secret = ###secret###

[6002](template)
fullname = kro smartphone
username = kros
secret = ###secret###

sip.conf

[general]
language=fr
bindport=19506
bindaddr=0.0.0.0
srvlookup=yes
canreinvite=no
defaultexpiry=3600
registertimeout=30
registerattempts=0
disallow=all
allow=ulaw
allowguest=yes
nat=yes

register => 0123456789:###secret###@sip3.ovh.fr

[to-ovh]
disallow=all
type=friend
secret=###secret###
host=sip3.ovh.fr
fromdomain=sip3.ovh.fr
fromuser=0123456789
username=0123456789
nat=yes
context=from-ovh
insecure=invite,port
qualify=yes
dtmfmode=inband
allow=ulaw

rtp.conf

[general]
rtpstart=41001
rtpend=41101

I have opened UDP ports 19506 and 41001 to 41101 on my firewall, and I can connect with my both devices on 6001 or 6002@sip.mydomain.com:19506

What am I doing wrong ? Is there a logfile that would explain why there is no sound ?