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.
Leave a specific request. What do you want your listener to do? Sure, you want them to call you back, but why? To answer a question? To set up an appointment? People will appreciate it if you give them specific actions for their call back. That way they’ll know they won’t be wasting a lot of time on the call back trying to figure out what you want.

It’s a good practice for each of your team members to have their own personal business phone numbers. There are many reasons why they shouldn’t use their personal cell phone number for business, so you’ll want to give them their own phone number through your VoIP provider or phone system.
All of these questions are pertinent to their call, and it’s important that you answer any that is relevant to your specific situation. Make sure not to drag on too long going through all of the info.