Office...originate...who cares. The key is to know the difference.
The hard part it looks like is taking the SIP and making it a analog for their old POS system. I'm not sure how you would do both in the same box.
Slow migration would be tricky with Analogue (ISDN is fine) with a POT's line there is no signalling, the call is delivered directly to the line itself and then it rings.
If you have an exchange line but multiple AUX (same number, multiple lines) they would all need to be migrated simultaneously.
I think that your best options are to either get a Patton gateway with an equivalent number of FXO ports or get the 3CX system prepared with a SIP trunk and then get the telephone number ported to the SIP trunk.
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