Namba ya mpokeaji ni namba isiyo sahihi.

Because the possessive linker agrees with the thing being possessed, not with the possessor.

Here, the thing being possessed is namba, and namba belongs to noun class 9 in the singular, so the possessive linker is ya.

So:

• namba ya mpokeaji = the recipient’s number
• not wa mpokeaji

If the possessed noun were different, the linker would change:

• jina la mpokeaji = the recipient’s name
• barua ya mpokeaji = the recipient’s letter

Mpokeaji means recipient, receiver, or the person who receives.

It comes from the verb pokea, meaning to receive. The ending -ji is often used to make a person-noun, similar to English endings like -er in receiver.

So:

• pokea = receive
• mpokeaji = receiver / recipient

Its plural is wapokeaji.

Ni is the copula, the word used for is / are in sentences like this.

So the basic structure is:

• Namba ya mpokeaji = the recipient’s number
• ni = is
• namba isiyo sahihi = an incorrect number

So the whole sentence is built like X is Y.

Swahili often repeats the noun in this kind of statement, especially in formal or system-style language.

So instead of saying only:

• The recipient’s number is incorrect

the sentence literally says something closer to:

• The recipient’s number is a number that is not correct

That is why you see:

• namba ya mpokeaji ... ni namba isiyo sahihi

This repeated-noun style is very natural in notices, forms, and error messages.

Isiyo sahihi means that is not correct or incorrect.

So:

• namba isiyo sahihi = an incorrect number
• literally: a number that is not correct

This is a very common way in Swahili to express an English adjective like incorrect, invalid, or wrong.

Isiyo is a negative relative form that agrees with namba.

A useful way to understand it is:

• i- = agreement with class 9 noun namba
• -si- = negative
• -yo = relative element, giving the sense of which / that

So isiyo means roughly which is not.

That is why:

• namba isiyo sahihi = number which is not correct

If the noun class changed, this form would also change:

• kitabu kisicho sahihi = a book that is not correct
• watu wasio sahihi = people who are not correct
• namba zisizo sahihi = numbers that are not correct

It actually can be, if you want a shorter sentence:

• Namba ya mpokeaji si sahihi = The recipient’s number is not correct.

That is a perfectly natural alternative.

But in the given sentence, isiyo sahihi is not the main predicate. It is modifying the second namba:

• namba isiyo sahihi = an incorrect number

So:

• si sahihi works as the main statement
• isiyo sahihi works as a modifier inside a noun phrase

That is the key difference.

It is singular here.

A tricky point is that namba often looks the same in both singular and plural. The agreement words tell you which one it is.

In this sentence, singular agreement appears in:

• ya in namba ya mpokeaji
• isiyo in namba isiyo sahihi

If it were plural, you would expect plural class-10 agreement, for example:

• Namba za wapokeaji ni namba zisizo sahihi = The recipients’ numbers are incorrect numbers

So the form namba itself does not change, but the agreement around it does.

Because sahihi is one of many adjectives in Swahili, especially borrowed ones, that often stay unchanged.

The agreement is shown elsewhere, especially in words like:

• ya
• isiyo

So in:

• namba sahihi = correct number
• namba isiyo sahihi = incorrect number

the word sahihi stays the same, while the agreement appears in the surrounding grammar.

Yes. A shorter and very common version is:

• Namba ya mpokeaji si sahihi.

That means the same thing in everyday use: The recipient’s number is not correct.

The full version:

• Namba ya mpokeaji ni namba isiyo sahihi.

sounds a bit more formal, explicit, or like an error message from a system. Both are correct, but the shorter one is often more conversational.