Erika:
One more response from Google. Please see the bolded text:
Hello Joshua,
Thank you for contacting Google. My name is Brandon, I understand you have questions about compatibility between your Nexus 5 and T-Mobile Visual Voicemail . According to our records, this particular app is not compatible with your Nexus 5 because T-Mobile Visual Voicemail is not compatible with Android 4.4.2 Kit-Kat.
Though we'd like to be able to help, app compatibility is determined by individual developers, not by Google Play, and we cannot change the requirements app developers set. To make sure our users have a great experience with Android apps on Google Play, at this time, apps that are listed as not compatible may not be downloaded to your device via the Google Play Store app. If you have any follow-up questions regarding compatibility, I'd recommend contacting the developer of T-Mobile Visual Voice-mail - you'll be able to reach this developer at http://www.t-mobile.com.
Thanks,
Brandon
The Google Support Team