Hopefully this will help. I simulate incoming calls for an all analog development system as follows: I take a copy of the production dialplan and create the internal test extension I will use to generate the call. In the code below this is extension 300 in [from-internal]. When 300 is dialed, the call is routed to [testcontext] where it gets a CALLERID(name) and CALLERID(number) assigned. The call is then routed to the s extension in the [from-pstn] context which is typically the part of the dialplan that handles incoming analog calls. This is used to test call flow, but it would be easy to adapt it to load test. Generating multiple calls could be done by sending the initial call in [testcontext] to a script using the AGI interface where the script contains a loop that alters the CALLERID(name) and CALLERID(number) and returns a series of calls to the s extension (or in your case to your MRCP server) in rapid succession. You could also have the script add any content you want to test with the MRCP server.
If you did not want to use a handset or endpoint to start your call you could keep the channel originate method.
[from-pstn]
exten => s,1,Verbose(INCOMING CALL DIALPLAN)
same => n
same => n
[from-internal]
exten => 300,1,Goto(testcontext,300,1)
[testcontext]
exten => 300,1,NoOp()
same => n,Set(CALLERID(number)=1234567890)
same => n,Set(CALLERID(name)=“NewCaller”)
same => n,Goto(from-pstn,s,1)