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pagerduty alternatives
service providers, hosted

    http://aws.amazon.com/sns/pricing/ (FREE TIER: Each month, Amazon SNS customers receive 1,000,000 Amazon SNS Requests, 100,000 HTTP notifications, 1,000 email notifications and 100 SMS notifications for free.) - see also: https://aws.amazon.com/mobile/?nc1=f_dr
    http://nodeping.com/ (Monitor 5 sites or services for $8/Month unlimited messaging (sms), ...)
    https://victorops.com/ ($34 /user/month billed monthly - 500 api calls - unlimited international sms)
    twillo.com
        https://www.twilio.com/sms/pricing/de (SMS to local mobile numbers: 0,06 EUR, local mobile number 2,66 EUR / month)
        https://www.twilio.com/sms/features (for example: Use Twilio Programmable SMS with your existing SMPP applications.)
        https://www.twilio.com/docs/tutorials
        https://www.twilio.com/docs/quickstart
        https://www.twilio.com/docs/libraries/ruby
        https://twilio.github.io/twilio-node/
        https://github.com/twilio/twilio-ruby
        https://www.twilio.com/monitor
    https://www.applozic.com/
    https://www.opsgenie.com/ ($17/user month-to-month STANDARD, 10 sms notifications / user,  additional sms: $0.35 per notification. ENTERPRISE: $29 month-to-month / user, unlimited sms) 
    http://www.everbridge.com/products/it-alerting/
    https://www.textmagic.com (see also: https://www.textmagic.com/free-tools/, €0.080 / sms, text2speech: €0.073 / msg, inbound mobile number Germany €7.00 / month)
    http://www.derdack.com/who-we-help-solutions/enterprise-it-ops/
        http://www.derdack.com/how-to-buy/contact-derdack-sales/ (one time perpetual license 7093.74 EUR)
        http://www.derdack.com/free-trial/
    http://onpage.com/home/secure_messaging/enterprise-control-system-it/
    https://www.xmatters.com/
    http://www.derdack.com/enterprise-alert/reliable-mobile-alert-notifications/
    http://www.x-on.co.uk/service/alert-notification/alert.htm
    http://www.m2mcom.be/alerts-notifications/
    https://www.pushingbox.com/
    http://www.teamchat.com/en/case-studies/ops-alerts/

Hosted calendar sync, etc.

    https://www.cronofy.com/pricing/
        https://www.cronofy.com/developers/tutorials/getting-started/

pagerduty selfhosted alternative

    https://github.com/ustream/openduty (beta, selfhosted,

check_mk

    https://mathias-kettner.de/checkmk_mkeventd_actions.html

self hosted sms notificationsusing sip, (gsm modem) or gateway

    https://staging.mattfay.com.au/scripts/check_mk-pushbullet-notifications
    https://monitoring-portal.org/index.php?thread/29243-check-mk-und-smstools/
    https://github.com/opinkerfi/check_mk/tree/master/notifications
    https://github.com/opinkerfi/check_mk/blob/master/notifications/sms
    https://github.com/nicoh88/cmk_notification_sms-lox24
        https://www.lox24.eu/
    https://github.com/bbhenry/check_mk_server/blob/master/email-sms-custom.sh
    https://github.com/bbhenry/check_mk_server/find/master
    http://www.unrelatedshit.com/2015/02/22/xmpp-notifications-for-check_mk/
    http://www.barryodonovan.com/2007/05/19/nagios-sms-kapow
        http://www.kapow.co.uk/
    (smstools3) http://wiki.stizi.de/linux:nagios:sms-versand
    http://wiki.stizi.de/linux:sms-gateway
    http://wiki.monitoring-portal.org/nagios/howtos/smstools
    https://github.com/cfmjohn/check_mk
    https://github.com/tuxis-ie/nagast
    https://github.com/bashtoni/nagios-check-sip

asterisk

    http://www.voip-info.org/wiki/view/Use+Asterisk+for+Nagios+Notifications
    http://www.pplusdomain.net/asterisk_notification.html
    http://netbaud.blogspot.de/2013/01/nagios-phone-call-alerting-with-asterisk.html
    https://github.com/aastaneh/nagg

cli sip clients

    http://www.pjsip.org/
    http://search.cpan.org/~sullr/Net-SIP-0.687/lib/Net/SIP/Simple.pod

example solutions

    https://dzone.com/articles/logging-for-continuous-integration

reviews / comparisons

    http://usersnap.com/blog/top-saas-monitoring-tools-devops/
    https://www.getapp.com/it-management-software/a/opsgenie/
    https://www.getapp.com/it-management-software/a/opsgenie/compare/pagerduty/
    https://www.getapp.com/it-management-software/a/opsgenie/compare/victorops-vs-pagerduty/
    http://stackshare.io/stackups/pagerduty-vs-victorops-vs-opsgenie

VictorOps

We need to get the Enterprise account in order to use the "Transmogrifier" which allows us to bundle incident messages in order to minimize alerts. The Enterprise account is USD 49 / User when billed anually and USD 59 / User when billed monthly. If we setup 5 users and pay anually the price is USD 2940 / year. I have setup a test account. If we descide to evaluate VictorOps further, I will write a few routes and transmogrify incoming alerts to be paged to the operator on call. Since VictorOps charges exactly the same price as pagerduty does, I do not consider VictorOps as an alternative.
OpsGenie

On the first sight this Services Web GUI looks much "cleaner" than VictorOps or pagerduty. Notifications are triggered by E-Mail which can be completely customized so a check_mk implementation seems to be straight forward, see https://app.opsgenie.com/integration - a nagios Plugin is also available: https://www.opsgenie.com/docs/integrations/nagios/rich-integration

The docs hub - https://www.opsgenie.com/docs - has all the Information we need to setup a test. I will setup few notification routes and if it works, i'll try to use the nagios plugin.

The Pricing for the Enterprise Option is USD 24 / User / Month when billed annualy, USD 29 on monthly basis. Unlimited Number of API Calls and (international) SMS. The overall fee would be USD 1440 / year for 5 users.