Virtual SIM's are available from the suppliers listed on this site. You can connect typically via their platform or via an API (typically HTTP or SMPP).
Virtual SIM Hosting enables business looking for a long number to either use their own number (from a SIM card they have), or use an allocated mobile number (Virtual Mobile Number) from the supplier.
Choosing which option simply depends if you want a specific number or not and if you have the SIM card you want to use to store in the suppliers data centre.
Virtual SIM is another very popular alternative to SMS Short Codes. They are often faster to set up, span more coverage and are often more cost effective.