Each Istari had a different mission?

[Legate of Amon Lanc]

Originally Posted by Rhod the Red

Saruman was head of the White Council at the time, but Beren had no way of knowing whom Saruman really represented (the Valar). Nor
do we know why he cleared him access. While there's
the voluntary possibility, it's also possible he used his 'voice' power
on Beren too.

Saruman certainly was a charismatic figure from the start, so that surely might have played its role, but let's bear in mind that Saruman, at that point, was still "good", I would say as much good as it goes still, so he would not use his voice intentionally to manipulate the people. Probably. I mean, I would believe he wasn't still in the stage to "misuse power", or not on any conscious scale. It depends how you define his "voice", if you mean by it simply his charisma and personal rhetorical skill which he uses unconsciously, naturally, then yes, but I would not imagine he would "use Jedi Mind Trick" or "cast a Charm Person spell" (to borrow illustrative examples) for the sake of getting Orthanc. Remember also the situation: Gondor had lost its garrison in Isengard, it didn't have manpower to spare, giving it to an ally with the reputation of Saruman was almost certainly one of the best things to do even from the Steward's point of view. So no need for coerce or mind tricks.

[Rhod the Red]

so he would not use his voice intentionally to manipulate the people

That's speculation.

If we judge by how it performed in the chapter of
'The Voice of Saruman' it would have ideally
been used after Beren tells his servants to leave
so that Saruman's alone with him.

He wasn't 'good' then, he'd already said no
to an assault on Dol Guldor and if you check
the timeline at the back of TLotR he had already
searched the Gladden Fields, etc. Probably
already obtained the Ellendil Star from Isildur's
body. As well as at least got to the point of spying
on the Shire.

We don't know whether Beren saw the White Council
as allies. Presumably Beren could speak Elvish, like
all from the Numenorian line on Middle-earth. So it's
logical either Beren voluntarily gave him the keys to
Orthanc & Minas Tirith archives or he coerced him
when the opportunity came.

[Legate of Amon Lanc]

Originally Posted by Rhod the Red

That's speculation.

Of course it is, I said it's my personal point of view. But it's based on what we know about Saruman. He was still not corrupted by the thought about the Ring at that time, and for instance we know he still, for some time, had been discussing with Treebeard rather politely (asking him permissions if he could visit Fangorn at all and so on). He certainly would have wanted to have a base (since it was "cool" and gave him some more authority), but I think he would still be reluctant to "perform mind tricks" (see above my explanation of what I do and what I don't mean by him using his Voice), and, most of all, he would not even need to perform them.

He wasn't 'good' then, he'd already said no
to an assault on Dol Guldor and if you check
the timeline at the back of TLotR he had already
searched the Gladden Fields, etc. Probably
already obtained the Ellendil Star from Isildur's
body. As well as at least got to the point of spying
on the Shire.

That's not correct. Check the timeline. Saruman settled in Isengard at 2759 TA, he said no to the assault on Dol Guldur by 2851, which is also the year he started searching around the Gladden Fields (with the footnote that this was probably the time he started lusting after the One Ring). And obviously, it isn't even logical: he could not have started searching the Gladden Fields before he had a permanent base nearby, or found the remains of Isildur for that matter, since we know he had kept it in his treasury in Orthanc - which he would have to have in the first place. In any case, the timeline says it plainly and clearly.

We don't know whether Beren saw the White Council
as allies. Presumably Beren could speak Elvish, like
all from the Numenorian line on Middle-earth. So it's
logical either Beren voluntarily gave him the keys to
Orthanc & Minas Tirith archives or he coerced him
when the opportunity came.

We know it very well. In the Appendices, in the story of the Southern Kingdom, we read that after the Long Winter

...Gondor began to recover its strength. But Rohan was slower to be healed of the hurts that it had received. It was for this reason that Beren welcomed Saruman, and gave to him the keys of Orthanc; and from that year on (2759) Saruman dwelt in Isengard.

(emphasis mine, of course)

Another account of this is also in the part about the kings of Rohan:

It was at the crowning of Fréaláf that Saruman appeared, bringing gifts, and speaking great praise of the valour of the Rohirrim. All thought him a welcome guest. Soon after he took up his abode in Isengard. For this, Beren, Steward of Gondor, gave him leave, for Gondor still claimed Isengard as a fortress of its realm, and not part of Rohan. Beren also gave into Saruman's keeping the keys of Orthanc. That tower no enemy had been able to harm or to enter.
In this way Saruman began to behave as a lord of Men; for at first he held Isengard as a lieutenant of the Steward and warden of the tower. But Fréaláf was as glad as Beren to have this so, and to know that Isengard was in the hands of a strong friend.

We can hear the tone of wariness from the narrator, but that is only the narrator who knows (and the reader knows as well) what Saruman was going to become: but it is clear from the text that the rulers of Gondor and Rohan at that point thought of Saruman as a friend. Obviously.

There is another sentence in the following text, saying that Saruman "seemed to be a friend for long, and in the beginning he maybe was one in truth". The "maybe", true, makes it sound a little doubtful, but I think there is no need to think that he wasn't. (I mean, there is no "maybe" in the objective way of thinking; the "maybe" is merely the chronicler's note: but in the end, either Saruman was a friend, or wasn't. And I daresay he was, but what is even clearer is that there had been no doubt about him being a friend in the eyes of the Free Peoples - whether he truly had been one or not. In any case, you would not call him "maybe a friend" if he had been walking around mind-controlling people into giving him a fortress.)