Change number of rings iPhone 6s before voice mail picks up

Going to voicemail is not based on the number of rings, it is based on the time the call has been in progress. You can contact your cellular provider and see if they can increase that time, however understand that if they can it is very minute, it will not be much. Many of the providers cannot change it either, so do not be disappointed if they cannot do what you ask. This has nothing to do with the iPhone, all phones and carriers work this way.

That option would be controlled by your cellular provider. Contact them to determine if changes are possible.

Wrong forum

I've read several posts saying to dial *#61# to get a message, but all I hear is that the call cannot be placed as dialed.