Realtime extension context for incoming call

Hi, using version 1.6.2.8. I am using asterisk realtime extensions and it is working if i have context=default in sip.conf
general section and then

[default]
switch=>RealTime

in extensions.conf.

However i want to do this to distinguish between calls coming in from a different SIP provider

sip.conf

[sipgate]
context=sipgate

[vonage]
context=vonage

But how to set up extensions.conf to route an incoming callto Realtime because i need something like

extensions.conf

switch =>Realtime/context@extensions

Is there any channel variable that will give the context for an incoming call then i can keep one switch statement
in extensions.conf

Just to clarify i left off the other entries in a typical sip peer/user entry in sip.conf just for clarity

ie
[vonage]
host=
ip=
context=
etc…