Forensic linguistics has two meanings. One refers to the study of linguistic structure as it pertains to legal interpretation (i.e. testifying as to what a law says), and the other, the sense you have in mind, pertains to testifying as an expert about legally relevant facts at a trial. In the US, the relevant rule is FRE 702. There is no need for a special certificate, and most forensic phoneticians do not have certificates. The presiding judge will make a determination whether you are qualified as an expert. In practice, this comes down to showing that you do actually have relevant specialized knowledge, so you would submit a resume indicating the degrees, publications and practical experience you have in the area. Since judges aren't particularly tuned into details of the field, they tend to allow testimony from people who pass the basic sniff test for expertise. The bulk of the "work" in being a forensic phonetician is the scientific quality of their testimony, e.g. are you able to show that voiceprint evidence is so unreliable that it should not be trusted (assuming it is even allowed). It is my understanding that most FP experts are professionally in law enforcement, e.g. work for FBI labs; but, regular phoneticians might be contacted by an attorney.
"Certificates" are available from various companies, and might impress someone; one would be better off in an academic program from an established university (Cardiff for example). But they are not required.