Connecting 2 Sites

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Dear experts,

Hope you will be able to help me in solving this requirement

We have the following scenario.

Site A
1. Panasonic KX TDA 100D Digital PABX
2. 10 x Extensions
3. 3 x Analog trunks ( provision is present to add more analog lines )

Site B
1. OmniPCX Office RCE Medium
2. 10 x IP Extensions
3. 2 x Analog trunks.

Site A and Site B are connected via Fast VPN.

I want People from Site A to Call Site B and vice versa ,

I have planned to add few SIP licenses to the Alcatel and keep the PAP devices connected to Panasonic. but is it possible to configure 2 way connections?

What is the best practice to follow, What are the things I will need to buy ?

Please advise