Asterisk - OpenSER - ITSP setup

we want to setup an Asterisk service for multiple clients with 1 SIP trunk to our ITSP/PSTN provider.

We want to put OpenSER as front end and multiple Asterisk servers to handle the load for all the clients. We have 1 SIP trunk to our ITSP/PSTN provider.

How/where do we interconnect to out PSTN/ITSP provider? On the front-end OpenSER or do we need an OpenSER on the backend only for the SIP trunk?

How good is OpenSER with NAT?

OpenSer or SER both have good nat traversl support.

can you tell/advise me if we only need a frontend OpenSER server, or also 1 for the connection to our PSTN/ITSP provider?