What you are looking for is a Multiplexor based solution. With the routers in place, I do not think you can tunnel T1 transparently.
However there is another solution to your problem with out going for a new T1 line.
What you can do is, you can terminate your TDM voice Call leg on the Routesr at both the ends and create a VoIP call leg between the Routers to tunnel the Voice calls.
All that you may need is some simple configuration on the routers at both ends.
The configuration will look something like this.
R1
dial-peer voice 1 pots
incoming-called number
destination-pattern
direct-inward-dial
prefix
port
dial-peer voice 2 voip
destination-pattern
incoming-called number
session target ipv4:
R2
dial-peer voice 2 voip
incoming-called number
destination-pattern
session target ipv4:
dial-peer voice 1 pots
destination-pattern
prefix
incoming called-number
direct-inward-dial
port
Please let me know if you have any more questions.