HELP - using multiple IP Authenticated sip trunks from one PBX to route out over multiple IP's


i hope you guys can help,

basically we have an office building which we let out to clients, we are trying to implement elastix/freepbx to supply the tenants with pbx functionality in each office here comes the tricky bit.

We use gamma as our supplier and their sip trunks only allow IP based authentication, we want to use a seperate sip trunk for each tenant but the problem being with this gamma will be unable to determine which sip trunk the call has come from and will result in incorrect billing.

whilst it isnt a problem to get multiple IP’s assigned to my internet connection i need to be able to get each sip trunk to route over the IP i have assigned to that tenants sip account to be recognised at gamma.

so in summary.
Each tenant has own sip trunk and wan ip address which outbound calls need to route over
also if i can block extensions of different tenants ringing each other that would be great.

i.e tenant 1 extensions 100,101,102 can not ring
tenant 2 extensions 200,201,202

any help appreciated.

let me know if you need more info