after many tries I've come to the conclusion that escalations are the best way to get SMS notifications for nonworking hours.
By best, I mean I don't have to touch packs, hosts, services ( so no need to duplicate or modify any of them)

I've succeeded in making it work for host related alerts, but not for services yet.
Here is my setup (h=host / t=template / cg=contactgroup / c=contact / e=escalation / nw=notificationway )

\_ t/generic-host-sms.cfg
..\_ t/generic-host.cfg
\_ e/escalation-sms.cfg
..\_ cg/admin_by_sms.cfg
....\_ c/ATI.cfg
......\_ nw/sms.cfg

This is working well, and allows me to select in the host configuration files if it should trigger sms alerts at night, SIMPLY by adding the line:
use generic-host-sms
Great !

for services, I can't add generic-service-sms.cfg to my host, or I would get this error message in /tmp/shinken_checkconfig_result :
WARNING: [Shinken] [items] Host u'ati-test-ati' use/inherit from an unknown template (u'generic-service-sms') ! Imported from: /etc/shinken/hosts/ati-test-ati.cfg:1

As I'm using packs, I don't want to duplicate all services // or modify the files in packs (that get overwritten when packs get updated).
Using a servicegroup would be great (not rewritting nothing, just adding the escalation I want ) , but it does not accept this keywork...

Any idea on how to make work this cleanly ?