Is there any hardware VoIP PBX that allows external software to subscribe on its events, determine incoming caller IDs (including GSM numbers that have been gateway-ed) and perform outbound voice calls via API?
Thanks!
There are many PBX's which support Computer Telephony Integration (CTI).
The most common standards to support are CSTA and JTAPI - if you google 'PBX supporting JTAPI' for example you should see several examples in the first few pages of results.
Any PBX supporting these standards will most likely meet your needs.
It would be worth your while looking at Asterisk based PBX's also in case these met your needs and allowed more scope for further development/features if required in the future. Many firms make Asterisk PBX's - the following link provides a good list: