How can I use my phone as a MRT card?

I'm curious to know, how can I utilize my smartphone as a substitute for an MRT card? Is there a specific application or service that I can download or subscribe to, enabling me to seamlessly use my phone for commuting on the MRT system? Is there a need for any sort of registration or activation process? Additionally, are there any potential limitations or challenges I should be aware of when using my phone as an MRT card? I'm excited about the prospect of digitalizing my travel experience and would appreciate any guidance you can provide.