Perhaps you have used proto_smpp and have not encountered its shortcomings, but I think that its discussion is not very appropriate on this platform. I will briefly inform you that in this module there is no control over the results of sending commands neither for submit_sm, nor for enquire_link. For example, the absence of a response to enquire_link is not an error for him. It is also not possible to connect multiple SMEs with different credentials to one smpp-server. It seems that this module is in the MVP state, maybe I'm wrong.
Best regards, Vadim Saranov