To receive SMS, you need to purchase a number, setup the SMPP point to address to your SMSC and make sure you white list NeutrafiX (NTX) SMPP Service IP (126.96.36.199). Once that's done you can send a text message to the virtual number you purchased.
When the message comes in from the virtual provider it will be routed to NeuTrafiX, where it will lookup the number and the SMPP point to address and try to bind to them.
(GSM --> DID (Virtual Number) Provider --> NTX --> Customer)
For outbound, you need to have an outbound SMPP trunk that supports numeric sender ID, configure the src_address to the virtual number you purchased in order to have 2-way flow.
You can choose an outbound route of your choice that supports this and use it for termination. you can check outbound providers in NTX from Market View >> SMS to see if anyone is offering an SMS route with dynamic sender id to the destination you want to achieve two way SMS to.
(Customer --> NTX --> SMS Termination Provider --> GSM)