A lot of the functionality involved here depends heavily on the SMSC provider. There are no known issues with the support of SMSC by Pega. I see references by Pega to the SMSC specs.
The functionality appears best in NBAM 7.12 forward. From what I can see, there has been experience (testing by Pega and customer implementations) that shows multiple sessions, validity period and acknowledgements. Also, I see Pega rule forms capturing encoding settings as per this example.