Hook for inserting?

If you have access to the database, you could potentially create a trigger for that plugin-specific table. This bypasses the need for a hook. Depending on how they handle database schema updates, it might even stick around after a plugin update. I’ve never tried triggers on WP tables though, so take this with a grain of salt.

E.g.

CREATE TRIGGER after_customer_insert
AFTER INSERT
ON wp_plugin_customers FOR EACH ROW
BEGIN
    IF NEW.PhoneNumber IS NOT NULL THEN
        UPDATE wp_plugin_other_table SET column = NEW.PhoneNumber
        WHERE other_column = NEW.CustomerCompany;
    END IF;
END

Edit: You could also ask the plugin author about adding a hook. Sometimes they oblige.