Repeat notifications for users on-call?
There is no way to do that with default android and you will need to find applications in order to do that. For example in regards to text messages you can use an application such as GO SMS which has the option to set up a reminder that will set off a different ringtone/alert until you acknowledge the message.

I am sure there are different apps out there that will do this for other notifications such as email if you need it but it might take some work to find the apps you need.