(1) I phone her tonight.
(2) I’m phoning her tonight.
(The Cambridge Grammar of the English Language)
(1) and (2) are all possible expressions: (1) as a schedule or plan, (2) could be used in the same objects of (1), "but this is not limited to them: it could be that I have simply formed the intention to phone her (without consulting her or anyone else about the matter)”.
But for this following expression, ‘I’ll phone you tonight’, the book says “it’s hardly possible if I'd simply said, casually, I’ll phone you tonight.” What reason could be possible for this saying?
“The progressive is restricted to cases where human agency or intention is involved - hence the anomaly of examples like [ib: ‘The sun is setting at five tomorrow’]. The difference between non-progressive and progressive is fairly clear in pairs like [ii: (1) and (2) above]. The non-progressive suggests a schedule or plan: perhaps I regularly phone her on the first Sunday in the month, or perhaps the call is part of some larger plan or arrangement - it’s hardly possible if I’d simply said, casually, I’ll phone you tonight. The progressive [iib: (2) above] could be used in these schedule/plan scenarios, but it is not limited to them: it could be that I have simply formed the intention to phone her (without consulting her or anyone else about the matter) and am waiting till I think she’ll be in.” (CGEL, p.171)