This solution is described in section Message sending from web-cabinet.
This solution is usually added to existing features of Telephony, which is available to users.
To simplify let's arrange, that this feature will be provided to users for free.
1. Configure Message routing.
This is described in detail in section Configuring message routing.
2. Choose configured Route class:
3. Configure Inbound -> message billing pack = 'zero':
4. Add web-access to user (see instruction Providing web-access to user).