Thanks @Francewoha to mention ERPAL here and thanks to ChrisChinchilla for this great overview. This blogpost describes perfectly how the “Drupal way” should be used to build an integrated CRM solution with Drupal: Use entities to model your data and integrate it with views, rules and other contrib modules to change the behaviour of your CRM entities. This is the way RedHen goes and it is related to the way Drupal Commerce is build. There is also the Drupal module CRM Core as an entity based CRM framework that is very similar to RedHen from its architecture. There is another ERPAL distribution called ERPAL Platform. It is based on Drupal commerce and integrates Drupal Commerce with CRM Core and some other useful modules to have a framework distribution for flexible business applications. As it is not targeted to non profits, it provides you with features for the complete sales life cycle from CRM contacts and activities through to quotes, orders and invoices, all based on native Drupal Commerce entities. So ERPAL Platform targets Drupal developers and site builders to help them to build their own business applications with the most common Drupal modules.
The ERPAL for Service Provider distribution is an end user distribution for service providers that includes CRM, project management, timetracking, invoicing, reporting and many other features to manage a service providers business. It has lots of features for this use case but it is not as flexible as ERPAL Platform.
I hope this description gives the overview some addintional helpful information for people that want to start on top of a distribution instead of starting from scratch with plain entities.