I’ve had the same problem with voicemail not working (for three years in my case!) and no joy via 150.
This morning I remembered the way we used to change settings before smart phones, for things like call forwarding, withhold number and so on using MMI codes, otherwise known as USSD codes, when you use the phone keyboard to send instructions to the service provider using symbols and number combinations. My research turned up this solution, which worked straight away and my voicemail (and visual VM) now work for the first time in three years! This worked for my iPhone, I don’t know about Android phones, but it’s probably worth a try.
Just open the phone app and using the keypad, enter numbers, stars(✱) and hashes (#). The number shown (07953222222) is for EE’s voicemail, but you can enter any number you want to divert to. Don’t forget the hash (#) after the phone number.
1. To forward calls to Voicemail when you can’t answer, enter
✱61✱ then 07953222222# and tap the green Dial button
(To cancel, enter #61# and tap the green dial button)
2. To forward calls to VM when you are unreachable, enter
✱62✱ then 07953222222# and tap the green Dial button
(To cancel, enter #62# and tap the green dial button)
3. To forward calls to VM when you are busy, enter
✱67✱ then 07953222222# and tap the green Dial button
(To cancel, enter #67# and tap the green dial button)
If you want to check the status of these settings, enter ✱#61#, ✱#62# or ✱#67#
I hope this works for you too, good luck!
