More debug output:

phablet@ubuntu-phablet:~$ gdbus call --session --dest com.canonical.indicator.messages  --object-path /com/canonical/indicator/messages/phone  --method org.gtk.Menus.Start [0,1] | grep x-canonical-text
([(uint32 0, uint32 0, [{'action': <'indicator.messages'>, 'action-namespace': <'indicator'>, 'x-canonical-type': <'com.canonical.indicator.root'>, ':submenu': <(uint32 1, uint32 0)>}]), (1, 0, [{':section': <(uint32 1, uint32 1)>}, {':section': <(uint32 1, uint32 2)>}, {':section': <(uint32 1, uint32 3)>}]), (1, 1, [{'x-canonical-message-actions': <<[{'name': <'telephony-service-sms.msg-actions.323031332d31312d32375431333a35303a30302d303230302d33.quickReply'>, 'parameter-type': <'s'>}]>>, 'x-canonical-time': <int64 1385567400000000>, 'x-canonical-type': <'com.canonical.indicator.messages.messageitem'>, 'x-canonical-app-icon': <('themed', <['telephony-service-message-symbolic', 'telephony-service-message']>)>, 'x-canonical-message-id': <'323031332d31312d32375431333a35303a30302d303230302d33'>, 'label': <'27800'>, 'icon': <('file', <'file:///usr/share/telephony-service/assets/avatar-default@18.png'>)>, 'x-canonical-text': <'Alexander Sack has updated "december: the month of testing - phonedations testing standup", which is now @ Weekly from 13:00 to 14:00 on Monday from Mon Dec 2 t'>, 'x-canonical-subtitle': <''>, 'action': <'telephony-service-sms.msg.323031332d31312d32375431333a35303a30302d303230302d33'>}, {'x-canonical-message-actions': <<[{'name': <'telephony-service-sms.msg-actions.323031332d31312d32375431333a34373a31362d303230302d32.quickReply'>, 'parameter-type': <'s'>}]>>, 'x-canonical-time': <int64 1385567236000000>, 'x-canonical-type': <'com.canonical.indicator.messages.messageitem'>, 'x-canonical-app-icon': <('themed', <['telephony-service-message-symbolic', 'telephony-service-message']>)>, 'x-canonical-message-id': <'323031332d31312d32375431333a34373a31362d303230302d32'>, 'label': <'27800'>, 'icon': <('file', <'file:///usr/share/telephony-service/assets/avatar-default@18.png'>)>, 'x-canonical-text': <'Alexander Sack invites you to: december: the month of testing - phonedations testing sta... @ Weekly from 13:00 to 14:00 on Monday (<email address hidden>'>, 'x-canonical-subtitle': <''>, 'action': <'telephony-service-sms.msg.323031332d31312d32375431333a34373a31362d303230302d32'>}, {'x-canonical-message-actions': <<[{'name': <'telephony-service-sms.msg-actions.323031332d31312d32375431333a34343a35362d303230302d31.quickReply'>, 'parameter-type': <'s'>}]>>, 'x-canonical-time': <int64 1385567096000000>, 'x-canonical-type': <'com.canonical.indicator.messages.messageitem'>, 'x-canonical-app-icon': <('themed', <['telephony-service-message-symbolic', 'telephony-service-message']>)>, 'x-canonical-message-id': <'323031332d31312d32375431333a34343a35362d303230302d31'>, 'label': <'27800'>, 'icon': <('file', <'file:///usr/share/telephony-service/assets/avatar-default@18.png'>)>, 'x-canonical-text': <'Reminder: Phonedations Stand-up @ Wed Nov 27, 2013 14:00 - 14:30 in Hangout (<email address hidden>)'>, 'x-canonical-subtitle': <''>, 'action': <'telephony-service-sms.msg.323031332d31312d32375431333a34343a35362d303230302d31'>}, {'x-canonical-message-actions': <<[{'name': <'telephony-service-sms.msg-actions.323031332d31312d32375431323a34343a35322d303230302d31.quickReply'>, 'parameter-type': <'s'>}]>>, 'x-canonical-time': <int64 1385563492000000>, 'x-canonical-type': <'com.canonical.indicator.messages.messageitem'>, 'x-canonical-app-icon': <('themed', <['telephony-service-message-symbolic', 'telephony-service-message']>)>, 'x-canonical-message-id': <'323031332d31312d32375431323a34343a35322d303230302d31'>, 'label': <'29064'>, 'icon': <('file', <'file:///usr/share/telephony-service/assets/avatar-default@18.png'>)>, 'x-canonical-text': <'Mathieu Trudel-Lapierre has accepted Ofono CI Setup @ Mon Dec 2, 2013 13:00 - 13:30 (<email address hidden>)'>, 'x-canonical-subtitle': <''>, 'action': <'telephony-service-sms.msg.323031332d31312d32375431323a34343a35322d303230302d31'>}]), (1, 3, [{'action': <'remove-all'>, 'label': <'Clear All'>, 'x-canonical-type': <'com.canonical.indicator.button'>}])],)