Do I need any kind of Service from Service Providers

I am new to Asterisk and trying to know it better.
I want to use Asterisk for my CRM n IVR system.
Please recommend a right one for me.
I went through the whole documentation but I have a basic question.
Do I need to buy any kind of service from any service providers to use Asterisk;
Do I need to buy services like:
SIP Trunk
Hosting Service etc to use Asterisk as an IVR

Yes if you are using it for CRM, no, in general.

You can use use Asterisk for purely local calls without any service provider. You cannot connect to someone on the PSTN without one. The service provider can be the PSTN operator, or you can go indirectly through another provider.

Please read the External Connectivity chapter in