Unhappy [Q] S5 SM-900K voicemail number issues?

Hello all,

I have an issue with this model where I am not able to change the voicemail number in the settings. When I go to Call -> Settings -> Supplementary service -> Inbox -> Voicemail, it automatically dials *88 instead of 123 that T-Mobile uses. Does anybody have the same issue or know of a way to fix it so I can edit it? When I dial 123, it definitely goes to my voicemail but I am unable to use the T-mobile visual voicemail or even get notifications when somebody leaves me a voicemail. I enjoy the phone a lot but this issue is definitely a deal breaker if I am unable to get this working. Please help and thank you!