ulaw/alaw doesn't have anything to do with VM prompts.
You need to use the correct setting for the SD card you have which is U-law for US + a few and A-law for the rest.
Just change the User Locale
Locale: Default = Blank (Use system locale) icon small lock
Configures the language used for voicemail prompts played to the user, assuming the language is available on the voicemail server. See Supported Country and Locale Settings. On a digital extension it also controls the display language used for messages from the system. Note however that some phones have their own menu options for the selected language for the phone menus.
Companding LAW
These settings should not normally be changed from their defaults. They should only be used where 4400 Series phones (U-Law) are installed on systems which have A-Law digital trunks.
· A-Law or U-Law
PCM (Pulse Code Modulation) is a method for encoding voice as data. In telephony, two methods of PCM encoding are widely used, A-Law and U-Law (also called Mu-Law or µ-Law). Typically U-Law is used in North America and a few other locations while A-Law is used elsewhere. As well as setting the correct PCM encoding for the region, the A-Law or U-Law setting of a system when it is first started affects a wide range of regional defaults relating to line settings and other values.
· For IP500 V2 systems, the encoding default is set by the type of Feature Key installed when the system is first started. The cards are either specifically A-Law or U-Law. PARTNER Mode cards are U-Law. Norstar Mode cards are A-Law.
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