With SIM hosting a SIM card is hosted in a modem that makes contact with the mobile phone network just like an ordinary mobile phone connection. So an ordinary mobile phone subscription is required and all incoming and outgoing sms messages are sent through that number. This results in a very slow and unreliable transfer.  Even more important is the fact that most operators do not allow this, and certainly not with frequent use.

For our virtual numbers we have a direct link with the operators who immediately send on all traffic for our numbers. This allows sending faster and more reliable traffic. Besides the monthly costs of the virtual number, no extra subscription then needs to be taken out with an operator.