I use my FP2 with a Telekom-SIM (former T-Mobile) in Germany. Unfortunately the voice mail number (3311) cannot be saved under Phone - Settings - Call-Settings -SUBS Settings - Mailbox - Configuration - Voicemail number (translated from german). When trying, I get the error message “Error: The voice mail number could not be changed. If the problem persists, contact your provider”. As a consequence I cannot use the “1” on the dial pad to access my mail box.
Telekom says they cannot help unless you have bought your device there. After searching the web I found many postings describing the same problem with different devices and different providers, and one post really explaining the technical details: technical error description (german) .
If this analysis is right, Android tries to save the voice mail number on the SIM, but some types of SIM (Telekom, may be others too) declare the specific data field as not existing (for what reason ever), so saving fails.
My suggestion to Fairphone developers: enable FP2 to save the voice mail number in the phone in case the SIM doesn’t allow saving. That would enhance the independency from specific providers and their individual mysteries, and usability of the phone of course.
Perhaps just a workaround, but why don’t you just save “voicemail” as a contact?
Yes, that’s what most of the affected users currently do. Even dialing “3311” is not a problem as meanwhile I know this number very well Nevertheless it’s a bug which makes headaches to those who want to configure their phones. And the hint “…, contact your provider” is senseless because I am convinced they know about this problem.
The Telekom-Hotline suggested me to try my second, some years old SIM. So I tested it, but with no success. It’s very likely that Telekom (and perhaps other providers) configure their (newer) SIMs in a specific way due to the limited resources of this device or due to a restricted write policy to SIMs. Some of the phone / OS manufacturers have reacted and store the voice mail number locally in the phone. Most of the postings are related to Android only, and at the Android Issue Tracker there is a long list of cases.
The workaround “configuring the voice mail number with removed SIM” does not work in many cases because the settings are disabled when there is no SIM present, so at the FP2 too. The fact that this works on some devices is a hint that it is possible to store this number locally.
So I hope that this will be fixed in the future…