Talk Partner vs Port forwarding

Hello out there.
I thought my request was more common, but unfortunately both Zendesk support and their sales department can't answer directly to my questions.

The thing is:

At the moment we have an existing phone number and a pbx from a local provider.

The features I am looking for is the following:

- Handle incoming/outgoing calls at zendesk
- Ticket creation based on calls

Additional feature would be:
- Setup a PBX through zendesk instead of the PBX from our current provider

So. What is the best/cheapest way for me to get going?

What I understand about Talk Partner and Port forwarding is as follows:

• I have to subscribe to a Zendesk/Twilio Phone number, no matter if I use Port forwarding or having Zendesk Talk (Partner)

In terms of costs there are these differences if I'm right:

• Port forwarding
• Incoming calls - I will be charged by my current provider because I am forwarding each incoming call?
  
• Outgoing calls - I will be charged by Zendesk on the prices, there is listed
  
  
• Zendesk talk (or is it voice?)
  
• Besides the phone number subscription, I will be charges on both incoming and outgoing call by zendesk?
  

So once again my final question is:
What is the cheapest (easiest) way to handle calls at zendesk but keep the existing phone number :-)

Best Jonas