It is not illegal to record a telephone conversation if you are a party to the conversation and provided that you don't attach anything to or in the phone or its connections. Usually this can be used as evidence.
If you use a "bug" or something to intercept the phone line to record the call, you are in breach of statutory provisions and you cannot use it as evidence.
You can also use phone answering machine messages, and it is not illegal to record face to face conversations that you are involved in. If you are not a party to the conversation you are secretly recording, it is illegal and you cannot use it as evidence.
There are exceptions to this, such as obtaining a court warrant or by special members of the government or police.
Under evidence law, evidence obtained by illegal means may be admissible if its probable value far outweighs the prejudice brought to third parties. This means the court decides whether the evidence is relevant, goes to an evidentiary issue (i.e. a material issue that is necessary for proving or defending one's case) and whether it is of probable weight (i.e. whether the content and nature of the evidence will go far towards proving or defending one's case). Evidence obtained by illegal means may have credibility issues and therefore, even if they are admitted, may not be heavily relied upon by the court (or jury) given how it was obtained.
The short answer is: 1) it will likely be evidence obtained by illegal means unless there was a court warrant or statutory permission; 2) may be admissive with leave of the court.