OK. Here goes.
My experience has been with California tribes, but I have had a lot of conversations with members of other tribes & people who work with other tribes, and the experiences have been similar, een if the tribes are very different. So, here is what I would do. It may be a lot lot more work than you planned, but I think it will be effective.
First, there is a reason no one is returning calls. Indians prefer to deal with people, not company representatives they don't know. Until you set up some kind of relationship, you will have little success. In most other business situations, you can arrange for a meeting, have a bit of chit chat & get down to business. This won't work here. Well, you can do that, but you will find that your meeting is very unproductive, and still have the no phone call problem. As I mentioned, it would be best to have at least two visits. The first, a general meet & greet; the second more focussed.
The first thing I would do is call the main company. Find someone who has been working with the tribe. You want someone who has developed good rapport and a good working relationship. If you get a big sigh and "It's impossible to work with those people. They always stonewall." you are talking to the wrong person.
Once you find Mr. or Ms. Right, have a long talk (I'll call the person Chris Right). Ask them how the tribe works. Who are the important people for you to talk to. The important people may not be the ones who are important on paper. Sometimes the tribal chair and council are powerful; sometimes only figureheads to keep the Feds happy. It may look like you need to talk to the Environmental Director, but everyone knows that nothing happens until the vice tribal chair gives approval. Sometimes you need to be vetted by people you will never know are important, such as certain elders.
Chris also should help you get a feel for the culture of the tribe. What is sacred to them and therefore off limits to you. Are there subjects that you shouldn't bring up? Is it OK to praise children or not? Do most tribe memebers live on or off the reservation?
Once you've got a good idea, ask Chris Right to set up a meeting with the first important person you need to see. It might be the vice chair or the vice chair's assistant or actually the Environmental Director. Chris must set up the meeting for you. Do not try to call and say "Chris Right suggested I call you." Chris has to make the call and set it up for you. Chris also has to attend the meeting--this is critical. Chris is vouching for you.
You may be puzzled about some of the people who attend the first meeting. Assume that they are there for a purpose and treat them as business colleagues. They are probably there to check you out.
Remember "personal." Usually it's a bad idea to bring your personal life to a business meeting, but here it is not. Introduce yourself. Talk about your credentials and what you do for your employer, but also mention yourself and your family. "I have three kids." "I just got back fro a trip to England." "My grandson told me he wished he could come with me since he is interested in geology and the XY rock formation has always interested him." The idea is that you are visiting because you want to get to know the people with whom your company is working and you would like them to get to know you.
Listen. Listen more than usual. If someone goes off on a tangent, don't try to bring the meeting back to the main subject. There is a good chance you are being tested, to see how you react.
Expect that little may be resolved at that first meeting.
Be extremely deferential and respectful of elders, even more than you might be to your own grandparents. This is a very important thing in Indian cultures. Your behavior toward elders will be noticed. This means you may have to be observant--while you are trying to remember everything else! It doesn't matter if you have been introduced to the elder. Hold open doors, give up your seat, offer to bring a cup of coffee. On the other hand, don't strike up a conversation with an elder unless it's clear that it is OK.
Don't bring gifts to that first meeting. Gifts can be important, but, again, they need to be personal. A box of doughnuts or even homemade cookies are generic. (But do ask Chris Right if you should bring refreshments to the meeting. Refreshments, not a gift.) Wait until you get back, then send a gift. Remove the gift from the business context. The gift can be an inexpensive token, as long as it is personal. "We we were talking about cooking and I thought you might enjoy this cookbook of specialties from my home town." "When my son was 3, he loved this toy. It sounds like your son would as well." "The office staff mentioned how much they like flavored coffees. Here is my favorite one from my favorite shop."
Your gift may or may not be acknowledged. In some cultures, it is not supposed to ever be mentioned. But they will remember.
I found that Indians will accept a genuine and sincere "I don't know" as an answer. In fact,they will prefer it to vagueness and equivocation. It took me along time to realize that "I don't know" would not elicit an angry "what do you mean you don't know?!!" Offers to find the answer and report back must be folowed up on, or you will lose a lot of goodwill.
I would talk to your boss and Chris Right and be prepared with a response to "what do you think of the pipeline situation?" There is a good chance you might be asked. You may want to say, sincerely, that it is really complicated, and the more you look into it, the more complicated it seems ot get,but you are trying to keep an open mind about all sides. But that is something you need to work out for yourself. Just be prepared for it.
OK. I'll stop now. Let me know if something doesn't make sense or if you have other questions.