Samsung Galaxy S6 — Voicemail setting grayed out, cant changr it.

Hi,
I've Tmobile galaxy s6 egde plus unlocked phone. I bought it from friend and have benn using it since last 3 years. I used the simple mobile carrier for last three years and i think they use the same number for voicemail as Tmobile. But now, i switched to lyca mobile carrier and their voicemail number is different. Whenever I click the voicemail button it gives me a message that "there is jo voicemail number in sim" I tried to change it through voicemail setting but it's not letting me do that because its disabled or grayed out. I've a andriod version 7.0. Please help me out because everytime I've to dial the voicemail number to check if i get a voicemail from anyone or not.
Regards

Thanks I'll definitely do the stock rom thing. I heard about it. But is thwre any better way you recommend me to reinstall the stock rom.