Consult your PBX vendor documentation.  

The following are third party references providing documentation regarding CSR generation for common platforms.  As Voyant certifies additional endpoints, we will update our help section with full configuration guides.

FreePBX Certificate Management
https://wiki.freepbx.org/display/FPG/Certificate+Management+User+Guide

Freeswitch
openssl req -new -key call.(domain here).key -out call.(domain here).csr 

openssl genrsa -out call.(domain here).csr 2048

Cisco CUBE CSR Generation example
https://community.cisco.com/t5/collaboration-voice-and-video/configure-sip-tls-using-ca-signed-certificates-between-call/ta-p/3164144

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