Can an unlocked iPhone 4s work without any issues on simple mobile carrier?

You should ask the intended carrier. I use tmobile when I am in the US and not all features work but they did tell me this when I purchased their Sim card. With the carrier I use at home all features work well but I know other carriers at home who do not provide Visual Voice Mail or Hotsport services.

I have an iPhone 4S (at&T) working with RedPocket network wich is the same coverage than at&t but prepay, it works perfectly but the MMS feature works only if you Jailbreak your iPhone and this can get done easy, free and quick if you look for it on YouTube. Actually the 4G works so the phone is super fast and for only $60/mo is a great deal!!🙂

Simple uses the T-Mobile network, Since the iPhone 4S doesn't support the 1700Mhz band, data will only be provided at 2G (EDGE) speeds (really slow). Visual voice mail will probably not be avaiable.

http://www.apple.com/iphone/specs.html

http://www.mysimplemobile.com/Simple-Mobile-Phones-Approved.aspx