>>When picking a provider, some plans talk about supporting a certain number of 'channels', which I gather is the number of simultaneous calls. One channel would mean that callers get a busy signal if you are already on the phone. Two channels would allow call waiting, or going to VM in that situation.
Correct.
>>Some plans mention that 2 users/SIP accounts are included. Does this mean 2 phone numbers are supported, or two different billing statements, or?
Sub-accounts are sometimes included. What does that do for you?
You get an account with a provider to use their service (1 billing statement). That typically includes 1 configurable voice service/SIP account. It may include additional sub-accounts as additional configurable voice services/SIP accounts. You typically need 1 voice service/SIP account for each individual end user/device... set of phones on 1 'line', 1 softphone on a mobile device, etc.
The OBi202 can be configured for 4 voice services/SIP accounts and 2 end user devices/phones/lines... you'll use 1 voice service/SIP account for each PHONE port to have 2 different phone 'lines'.
A DID can ring one or more voice services/SIP accounts... 1 DID can ring all SIP accounts; 5 DIDs can all ring the same 1 account. You can wire it up how you want it, depending on the provider and practical usage. The typical configuration is to mimic traditional phone service and go from there: did1 >> SIP account1 >> phonejack1.
A voice service/SIP account can carry unlimited SIP sessions/channels subject to the rate plan. So, 1 voice service/SIP account can have many calls in progress, subject to the channels and bandwidth available, and the physical practicalities of the endpoint... an OBi202 with 2 PHONE jacks can only connect 2 users/lines/sets of phones.
>>Is there any technical difference between a local DID number I get from a provider, and my ported landline#?
No.
>>Generic (ookla) voip tests show I am getting > 25mbps, 20ms pings, 5ms jitter and no errors. Are there known ISPs that the Obi ATA does not work well with due to voip traffic throttling or other issues?
Probably. You'll have to try it to find out. Your router is also significant... it must pass SIP traffic without fault.
DIY BYOD VoIP makes it a hobby. VoIP is not POTS. If you go this route and set it up, you will be the one supporting it on your network. Once established, it can be as reliable as any other computing device on your network using an online service 24x7.