I have worked on PerfexCRM (https://perfexcrm.com/) Custom Modules development and Changing Fields/ Adding new modules and features. Using the PHP CodeIgnitor Framework, PerfexCRM is a highly customisable CRM.
1) Add Modules.
2) Integrate API.
3) Customise Lead, Proposal, Estimate, Invoice, Project, Web To Lead Form, Report, Dashboard, Customer, Contact, Ticket, Staff & Many more modules.
4) Customise the UI/UX
If you have any requirements please send message.