I could see an argument for relaxing that for mobile voicemail, on the basis that mobile phones are pretty personal and not generally shared by multiple people, but I've no idea whether that may have happened. Or, indeed, whether such policies may vary between organisations within the NHS.
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.

When creating an individual voicemail greeting, clearly identify yourself, your role, and your company, and let the caller know when you will be able to return their call. Here are a few voicemail greeting scripts you can use with your personal work phone extension: On Another Call Example Script. Hi, this is [YOUR NAME] from [COMPANY NAME].
9. Update Your Greeting Frequently. Many of the things that can make your voicemail great, also require that you update it pretty regularly. Information like deals you’re running, when you’re away, and who to contact while you’re out can take your greeting to the next level, but they’re obviously time sensitive.