Mobile service provider Talk 'N Text has this Missed Call Alerts (actually called Miss Call Alerts on their SMS confirmation) feature that redirects your callers to voicemail when you fail to answer their call. If they leave a voice message, they will be charged their regular or promo call rates. You will then get a text message notifying that you have voicemail and retrieving it will be charged P1.00 per call.
To turn on this feature, text VM ON to 8990. Once you are subscribed to this service, your missed call alerts via SMS will come from 8993 + the mobile number of the person who left a voice message (ex. 8993639123456789). You will need to dial *799218 to retrieve your voicemails.
Now, to turn off this feature, just text VM OFF to 8990. You will then get an SMS saying that you will no longer receive SMS from this service. Callers will no longer get redirected when you can't answer their calls.
Note: We wanted this feature turned off (we don't remember subscribing to it), and had to endure lengthy calls with *888 until a finally capable CSR gave this information. Now I'm actually surprised that there is nothing about this on their official website, nor did a Google search bring a useful result. Posting this to potentially help others wanting to opt out (or opt in) of this service as well.