In Australian English it’s pronounced with the vowel /a:/ like in ‘part’. Problems arise when people use the /ʌ/ vowel (like in ‘up’) instead of /æ/ or /a:/. If you do this is will sound like the worst swear word in English. Many non-native speakers often pronounce the vowel /æ/ more like /ʌ/ because they don’t have a vowel like /æ/ in their first language. Many speakers of European languages will do this (Spanish speakers and Italian speakers) and also speakers of Japanese and Korean. This problem with /æ/ also means that if you say the word ‘back’ in your voicemail greeting sample, you are likely to pronounce it more like ‘buck’. remember to pronounce word endings in English. Check you aren’t dropping any endings off or mispronouncing them.
You have reached xxx-xxxx. We picked this machine up at a garage sale in “as-is” condition. You can try to leave a message on it, but we are not sure it will be recorded. If we don’t return your call, it means the machine did not work.

Simply speak into your phone or computer while recording a greeting with your business phone service. If you need to create your own audio file, the voice memo app on your phone should do fine.
Just in case you fall victim to mumbling or speaking softly in your voicemail message be sure to repeat your contact information at the beginning AND end of your message so that the listener is more likely to be able to pick up on the information. This helps if you are speaking quietly or if the listener was not ready to record your information when you said it the first time. Repeating your information will save the listener time as they will not have to listen to your message again to get those details.