VoIP - Conference Calls
Whilst not a feature on the our server side, conferencing multiple calls together is possible on many VoIP phones. E.g., the Snom series has a 'conference' button which enables multiple calls to be joined together. Please see the documentation for your phone regarding this.
Maximum number of calls
The VoIP service doesn't limit the number of calls you can make, however different phones have a different number of maximum calls you can conference together. Some phones may only be able to conference 3, whilst others can conference more. Check the technical features of the phone for the details.