+0
Hello.
I have came across two uses of will that seems interchangeable: willingness to do something (offer) and a decision made at the moment of speaking, so
Do you consider these sentences as an offer or as a decision made at the moment of speaking?

A: The phone is ringing.
B: I'll get it.

A: This bag is heavy.
B: I'll carry it for you.
Comments  
If I may give my opinion, I think that both examples are correct.

Sextus
You are absolutely correct. Both examples show how the ideas of offering (expressing willingness) and of deciding spontaneously to make the offer coalesce into a single kind of usage of "will".
On the other hand, sometimes we say (spontaneously) "I'll do it" not out of willingness or out of a desire to offer our services, but rather out of frustration in a situation where no one seems able to decide how 'it' is going to get done! Then, it's like "I'll do it, (or it won't get done)"!

CJ
Students: Are you brave enough to let our tutors analyse your pronunciation?
Latin, I have a feeling that these two examples are not getting at the difference between the two uses that interest you here. Both of your examples relate more to making an offer; (of course, one must first decide to make such an offer). It seems, however, when using will in the sense of 'a decision made at the moment' (I don't quite know where you found that particular definition), it may be referring to 'willing something to happen'. That is to say, by the force of one's determination, (the power of the will), an action is to be accomplished.
Davkett It seems, however, when using will in the sense of 'a decision made at the moment' (I don't quite know where you found that particular definition), it may be referring to 'willing something to happen'.

When I formally studied English, I read and was told that this use of "will" is identified as "instant decision", in contrast to a decision one has already made. In this case one should employ "going to".

Sextus
DavkettLatin, I have a feeling that these two examples are not getting at the difference between the two uses that interest you here.
Yes, maybe the examples overlap each other. Now, do you think these make it more clear this subject?

I'll lend you some money. (offer, not decision)

These trousers are perfect. I'll take them. (decision)
Teachers: We supply a list of EFL job vacancies
OK. I think, from these new examples, that I understand your point better. One is propositional, the other decisive. I wonder if this sort of difference is as much in the inflection of the voice as it might be in the larger context of the sentence.

Example:

I will lend you some money.
I will lend you some money.