Bridging a Nortel and CISCO??? NY Engineer needed.Nortel

We are trying to setup VOIP from our Nortel BCM 5.0 phone system to our other offices in London and HK, all who have Cisco networks.
Firstly, is it possible to bridge the two so that they can talk to each other? My telco is only a Nortel provider and does not seem interested in helping.
Secondly, does anyone know a company in NY or surrounding areas who have the expertise/knowledge to make this happen?
Many thanks