If you need help to call somebody.

this sentence is indeed only a fragment of a full sentence.

Compare:

I need help to find a telephone. (What kind of help do I need? For what reason do I need help?)
I need help to find a car. (I cannot find a car on my own)
I need help to call somebody. (I cannot call somebody on my own. In order to call somebody, I must be helped by someone or something)

You can disassemble the first sentence in this way:

If you need help, call somebody. ->
Do you need help? Call somebody! (roughly the same meaning)

But you cannot disassemble the second (incomplete) sentence in a similar way:

If you need help to call somebody. ->
Do you need help? To call somebody! (very weird-looking combination of sentences)

"Call somebody!" is an imperative sentence - it could be a command or a request, and in combination with an if-clause we can have a conditional sentence:

If you need help, call somebody"

"To call somebody!" is just an exclamation when used on its own. It does not work as an imperative sentence. And when used in a bigger sentence, it becomes an infinitive clause, which also cannot work as a command or request, so we cannot use it as an apodosis in a conditional sentence.

If you need help to call somebody.

this sentence is indeed only a fragment of a full sentence.

Compare:

I need help to find a telephone. (What kind of help do I need? For what reason do I need help?)
I need help to find a car. (I cannot find a car on my own)
I need help to call somebody. (I cannot call somebody on my own. In order to call somebody, I must be helped by someone or something)

You can disassemble the first sentence in this way:

If you need help, call somebody. ->
Do you need help? Call somebody! (roughly the same meaning)

But you cannot disassemble the second (incomplete) sentence in a similar way:

If you need help to call somebody. ->
Do you need help? To call somebody! (very weird-looking combination of sentences)

"Call somebody!" is an imperative sentence - it could be a command or a request, and in combination with an if-clause we can have a conditional sentence:

If you need help, call somebody"

"To call somebody!" is just an exclamation when used on its own. It does not work as an imperative sentence. And when used in a bigger sentence, it becomes an infinitive clause, which also cannot work as a command or request, so we cannot use it as an apodosis in a conditional sentence.

If you need help to call somebody.

this sentence is indeed only a fragment of a full sentence.

Compare:

I need help to find a telephone. (What kind of help do I need? For what reason do I need help?)
I need help to find a car. (I cannot find a car on my own)
I need help to call somebody. (I cannot call somebody on my own. In order to call somebody, I must be helped by someone or something)

If you need help to call somebody.

this sentence is indeed only a fragment of a full sentence.

Compare:

I need help to find a telephone. (What kind of help do I need? For what reason do I need help?)
I need help to find a car. (I cannot find a car on my own)
I need help to call somebody. (I cannot call somebody on my own. In order to call somebody, I must be helped by someone or something)

You can disassemble the first sentence in this way:

If you need help, call somebody. ->
Do you need help? Call somebody! (roughly the same meaning)

But you cannot disassemble the second (incomplete) sentence in a similar way:

If you need help to call somebody. ->
Do you need help? To call somebody! (very weird-looking combination of sentences)

"Call somebody!" is an imperative sentence - it could be a command or a request, and in combination with an if-clause we can have a conditional sentence:

If you need help, call somebody"

"To call somebody!" is just an exclamation when used on its own. It does not work as an imperative sentence. And when used in a bigger sentence, it becomes an infinitive clause, which also cannot work as a command or request, so we cannot use it as an apodosis in a conditional sentence.