Hello,
You can hide a lot of things in epesi without much hassle. It's worth noting that in the upcoming version 1.3 will include features that will make it much, much easier.
Changing field type to hidden is not the way to go about hiding fields. This carries major consequences to how EPESI CRUD handles these fields and it may and will cause errors. Moving these fields to a tab is a solution that will work well, but on the user-end these fields will still exist.
In the current version, you'd have to hook up a new permissions callback and change the way permissions are handled. First of all, remove the "permission" field from checking who can access what records and second - hide specified fields in view mode. You could also completely delete these fields, but then there's a risk it will break core functionality, possibly resulting in errors.
In the new version, you can simply edit what fields should be hidden to user using admin backend. You can also easily remove check for the permission field to determine access.
Module Phonecalls can be safely uninstalled. To do so go to Menu->Admininistrator, Modules Administration, First uninstall "Utils/SetDefaults", then "CRM/Phonecall". This will get rid of "Create new phonecall" icons as well.
Companies - that's a more problematic set. It's strictly linked with contacts and it's packed in the same module, so uninstalling companies is out of question.
It's not recommended to remove this particular module because it's likely to cause more trouble than bring gain. Theoretically, in the current version, it is possible to modify the code to achieve that. Open modules/CRM/Contacts/ContactsCommon_0.php, find function menu() and remove it. Then in contacts hide field company name. Still, super-administrator should be able to see the field, since new users created in epesi should have a contact assigned to Main Company. That's what's distinguishes contacts between Employees and Customers.
As for the Roundcube (e-mail client) - unfortunately we tied it a bit too tightly to Tasks and Meetings. Since these are needed, I recommend modifying the code of modules/CRM/Roundcube/RoundcubeCommon_0.php and remove function menu().
In the upcoming version 1.3, it'll be possible to hide these entries from the users by simply taking browse (view) access to respective recordSets using admin backend.
Kind regards,
Arek