iPhone Voicemail function

Hey everyone!

I recently signed up to Smarty, all ok so far, however, when I receive a voicemail I get a text message from Smarty informing me there is a voicemail and to call the number in the text to listen to the voicemail. That's all well and good but with my previous mobile provider and as Apple have designed it, voicemails should appear in the voicemail tab within the phone app on my iPhone. I used to see the voicemails in a list and could play them from there, fast forward, rewind etc. 

Is there a way to set this up to work again with Smarty? I can't find any info on the help pages anywhere or in this community? Any help would be super cool.

Thanks!!