Ok, here is the issue and problem.
If you just had to make a few calls - get say weather, or say do a calls for a SMS like you did?
Then I am a "big" fan of using MSXML. It is nice, and if the structures and data is not too complex, then you can make a REST call or even a SOAP call.
The problem with EchoSign (or whatever they are calling these days)?
You have a nice "list" of things to do, and I think hand coding out the calls from Access would be quite hard.
And they do have a WSDL for their signing software. (at least when I looked at and used it with Access about 7 years ago they did). Now most of the industry has moved towards REST (as opposed to SOAP calls). However, the API's to Echo sign are a bit on the complex side. Because there is "some" complexity here, then I do suggest writing the interface in .net, and then exposing the .net code you write as a OBJECT to Access/VBA code.
So for a few REST calls? Sure, MSXML is the way to go. And if you take the XML headers (the get and post headers they provide), you could perhaps cobble this together in Access.
However, because they provide WSDL's (if you choose to use soap) and they provide the documentaiton for .net code, then building up what you need and want is OH SO much less work and efforts in .net.
So, you write the bits and parts in .net, and then expose a "inter-op" object from .net for use in VBA.
Now, unfortantly the above thus assumes you have familiarity with .net, and more so have on occasion used .net code from Access. Both of these things are not all that hard, but of course if you not done this in the past, then you have a good deal of learning.
So, now you having to:
Trade the time and effort of using MSXML + 100% VBA.
vs
Learning .net, using existing published API's they have for EchoSign and writing out what you need. If you have the .net skills part, then without question that's the way to go. It much less work if you done this in the past in .net.
Edit: I just checked their side - SOAP + WSDL's was depreciated mid last year. So in the 7 years since I done this,, they dropped the exact path I was suggestion! (SOAP + WSDL's).
So, it is now REST calls all the way.
So, this being REST calls now? Well, that certainly bodes well here. This would thus suggest that using MSXML for this might not be all that bad. I was lazy, and had WSDL's (they will generate the vb.net code for you - so it rather easy to do things this way).
Given that they are now REST calls? Well, that means two things:
You have a lot of parameters that are part of the URL (just like any web url that has parameters).
And then you have often some "headers" that you have to setup.
Because their is a LOT of parameters, then I would probably want to build a header builder routine.
You can google VBA REST calls - you get lots of hits.
I don't have a active Adobe developer account anymore - so I can't really take a stab at how much work this will be in VBA. But, you certainly can cobble this together in VBA.
As noted, if they still supported SOAP - then without question .net would and was the way to go. But , now without WSDL's to generate your .net code, and Adobe dropping SOAP support, then this will be quite a bit of work - (in .net or VBA).
I don't have more to offer then above. I started typing this, since I built a Adobe interface to Access/VBA using SOAP, but now that "easy road has been removed as a option.
I not attempted to code out a complex rest interface in .net or VBA, and I don't think the work is now much different in terms of using VBA or .net. But without a SOAP option, then you have to code this out - and that's going to be quite a bit of work.
Sorry I can't help much more, but no one else jumped in - so at least now you know you CAN code this out - it just a question of getting up to speed in making REST calls from MSXML. Google around - perhaps some samples exist for signing from VBA. I don't know what code samples they now provide, since as noted, it been about 7-8 years ago since I did this (in .net).
Albert D. Kallal (Access MVP 2003-2017)
Edmonton, Alberta Canada