The number of recalls necessary is defined in the rules for each type of loan:
Settings > Define core data > Rules > Edit
Here you also define the interval between recalls.
The preferred channel (Mail, text message, etc.) for sending out recalls is defined for each user group:
Settings > Define core data > User Groups > Edit
You may also define the preferred channel, or method, for each individual recall:
Settings > Interface > Define Notifications > Edit
There is a "Hierarchy" among the channels, or methods, used in recalling items.
If text message is chosen as preferred channel for first recall of an item, and there is no available phone number for the patron, an email is sent instead.
If no email address is available, a letter is printed.