Se vi ne respondas al mia mesaĝo, mi telefonas al vi.

Se means if. It introduces a condition:

• Se vi ne respondas al mia mesaĝo = If you do not reply to my message
• mi telefonas al vi = I call / will call you

In Esperanto, the se-clause can come first or second:

• Se vi ne respondas al mia mesaĝo, mi telefonas al vi.
• Mi telefonas al vi, se vi ne respondas al mia mesaĝo.

Both are grammatical.

Esperanto sometimes uses the present tense for something that is regular, expected, or understood from context, including a near-future idea.

So this sentence can feel like:

• a general rule: If you don’t reply to my message, I call you
• or a very natural future idea: If you don’t reply to my message, I’ll call you

If you want to make the future especially explicit, you can also say:

• Se vi ne respondos al mia mesaĝo, mi telefonos al vi.

That version is more clearly future.

Yes. That is a very common and clear future version.

Compare:

• Se vi ne respondas..., mi telefonas...
  more like a general policy, an immediate plan, or a vivid present-style statement

• Se vi ne respondos..., mi telefonos...
  more explicitly future

Both are good Esperanto; the choice depends on the nuance you want.

In this sentence, respondi al means to reply to something.

So:

• respondi al mia mesaĝo = reply to my message
• respondi al vi = reply to you

The preposition al often marks the person or thing that receives the reply.

Because telefoni is normally used with al for the person you call:

• Mi telefonas al vi = I am calling / I call you

So in Esperanto, the person is introduced with al, not made a direct object with -n.

This is different from English, where call someone has no preposition.

Al usually means to.

In this sentence it appears twice:

• respondi al mia mesaĝo = reply to my message
• telefoni al vi = call / telephone to you

A native English speaker often wants to leave out to, because English sometimes does. Esperanto keeps al in these expressions.

Because possessives like mia, via, lia, ŝia, and so on usually take the place of the article la.

So you normally say:

• mia mesaĝo = my message

not:

• la mia mesaĝo

The second form is generally not the normal everyday way to say it.

In Esperanto, ne usually goes directly before the word it negates.

Here it negates the verb:

• vi respondas = you reply
• vi ne respondas = you do not reply

That is the normal word order.

Because it is part of a phrase with the preposition al:

• al mia mesaĝo

After a preposition, Esperanto normally does not use the accusative -n just to mark the object.

So:

• mia mesaĝo = nominative form
• al mia mesaĝo = to my message

No -n is needed here.

Yes, a comma is normally used to separate the se-clause from the main clause:

• Se vi ne respondas al mia mesaĝo, mi telefonas al vi.

That is the standard written style.

Vi can mean:

• you singular
• you plural
• and it is also used in polite/formal situations

Esperanto does not normally distinguish these in everyday usage. So from the sentence alone, vi could refer to one person or several people.

Mesaĝo is a general word meaning message. Depending on context, it can mean:

• a text message
• an online message
• a voicemail/message
• a message in a broader sense

If the context is mobile phones or chatting, many people will naturally understand mesaĝo as a text or chat message.

Yes, some changes are possible. For example:

• Mi telefonas al vi, se vi ne respondas al mia mesaĝo.

That still means the same thing.

However, inside smaller phrases, keep the important structure:

• respondi al mia mesaĝo
• telefoni al vi

So you should not remove al in those parts.