Calling certain numbers will result in silence instead of the normal ringing sound, known as the ringback tone. This silence will continue until the Voicemail message of the dialed number is heard. While this can be frustrating, this is due to the routing configuration of certain service providers, such as T-Mobile.
When placing a call to a T-Mobile device, they will first try to locate the device before returning the ringback tone you are accustomed to hearing. If the device cannot be located, due to coverage or equipment issues at the time, the call will then roll to voicemail without the ringback tone ever being sent. Per T-Mobile, this is working as designed on calls to their network.
While this can be confusing, especially during a day with back-to-back phone calls, this does not adversely affect the call in any way. The voicemail message will play and a voicemail can be left for the called party without issue.