Yes you are correct. VoIP-analog gateway would solve your problem. You connect your PSTN line to the gateway and make a SIP Trunk between the gateway and Asterisk. The gateway takes care of PSTN - VoIP conversion.
I am having excellent experiences with Patton SmartNode gateways.
Thanks for the suggestions - I’ll take a look at those.
@Thor - I considered support, but since it seems to concern support with asterisk, rather than general advice about interacting products, I felt general was more appropriate.