A registration for a different contact port number is a different registration.
To make this concrete, you could have both Asterisk and a soft phone on on your machine. They would have to bind to different ports for SIP. if you started with Asterisk registered to a particular address of record, and the soft phone tried to register to the same address of record. That would not be treated as a refresh of Asterisk’s registration, but rather a registration by the soft phone. The provider would either send all new incoming calls to the latter, or would start offering them to both.
So now the provider is allowing more than one registration. Either way they are still sending the Expire: 30 value.
You are getting that the trunk is requesting 1800 seconds, the provider is forcing it to 30 seconds and then complaining to the OP that registering every 20 seconds can get their IP banned.
Don’t matter if it is behind NAT or not, the provider keeps forcing the Expire to 30 seconds and then complaining about frequency of REGISTERs. They are causing the issue by ignoring the 1800 seconds they told the OP to use
What I’m suggesting is that they don’t understand the real problem, and what they are really complaining about is the number of changes, or, consequential on that, the number of credentials lookups. Unless they now continue complaining, I think you have to interpret it that they weren’t complaining about the total registrations, but rather about the number of registrations with changed contact addresses.
They are behaving like black box operators, by which I mean business people who buy and operate technology but don’t understand how it works, otherwise they would have easily seen that either specifying a 30 second was wrong, or that 20 second renwals wasn’t the real problem.