• Why are my A2P Messages towards Google Voice numbers rejected when sent with Long Virtual Numbers as Sender ID option but it is successful?
• Why are A2P messages successful for some US Broadband Numbers with Long Virtual Numbers are Sender ID but in other cases it is unsuccessful?

Resolution

Messages to the US Broadband Virtual numbers are delivered on best efforts and may not be guaranteed. According to the US Country Specific Features and Restrictions A2P Alert/Transactional messages would have to use either Shared Short code or Toll Free Number. However, T-Mobile and Sprint have heavy filtering on Shared Short Codes and hence have better delivery rate when the Messages are sent with 10 digit Virtual Number. The Long code i.e., 10 digit Virtual numbers are recommended is the recommended Sender ID option for US Broadband Virtual connected over T-Mobile and Verizon for the same reasons. 

The recommended Sender ID options for A2P messages to other Broadband Virtual Services such as Google Voice are either Shared Short code or Toll Free Number depending on the use case.