It's normally 24v here in the US. The fact that the signal is carried over cable is not terribly relevant. The output of the phone jack mimics what a standard pots line would deliver.
It's funny you mention that, though. The phone company used to adjust "ringer equivalence" to compensate for multiple phones in a home. I don't know if that was accomplished by increasing voltage or amps. My guess is voltage, because phone lines are very small, maybe 22ga at best.
Not long ago, I had a customer who had seven old school phones. The cable modem could not provide enough power to make them all ring. He could talk on any of them.
At any rate, the modem output is 24v, very low amperage.

Amateur Radio Operators had a device called a "phone Patch" which interfaced their landlines to their transceivers, to allow audio fron the telco in and out of a two way radio conversation.
 
Check around for a local ham who might have one in his stash of gear, or look at a local ham fest( a swap meet flea market type meetup)
 

 
 
 

In the US the DC voltage on a plain old telephone service (POTS) line is 48VDC. When the telephone goes off hook it closes a switch which draws current from the central office, starting the whole process. In the really old days they would reverse the polarity of the DC voltage to create party lines.

The ringing voltage is between 80VAC and 90VAC, at 20 Hz, and the duty cycle was 2 seconds on, 4 seconds off. As a kid I was taught that ten rings took one minute... an odd way to time things!

Line voltages, and ringing voltages and cycles differ from country to country.

For the theatrical types - a ringing generator is easy to build, but lately I've used a 70V amplifier (an old Toa) and I drive it with audio files that I created to provide different ring cadences.

Thanks musicman for the heads up on the Rolls. I ended up with a Symetrix TI-101 circa something like 1989.....I guess these kinds of units have probably improved since then.
Here is the .pdf
 http://www.symetrixaudio.com/kb/TI101_ug.pdf
 
The phone input needs to see a phone unhook in order for it to work. The schematic shows that it needs to have a phone in a Parallel configuration and once the phone receiver comes "off the hook" the box should be in the circuit. The manual recommends the phone mouth piece be unhooked on the phone so the listener hears your mic instead, and that signal is fed back into the TI-101.
 
Robert I have Comcast Cable as my carrier. I bought a splitter.Thanks for clearing up that the second jack requires me to buy another line! You gentlemen know a lot more about phone lines than I do and I have come to the right place. Thanks for your help!
I don't plan to use any "old school" phones...instead I had planned to simply use the phone I already have but cover the mouth piece on the handset in my studio. From what Robert has said, I feel a lot better about what I'm trying to do concerning my cable/phone modem. If I have a cordless phone I should be able to call out with it and * hopefully* there will be enough voltage to unhook the TI-101......my biggest concern is that the load of the TI-101 might be excessive.

Keep in mind that when the TI-101 was a current product the rules concerning telephones and the telephone network were very different! The whole landscape was different! The cost to have a device "type accepted" for direct connection to the public telephone network was beyond that which could be amortized even if every broadcaster purchased one!

So companies skirted around the issue by printing in their manuals that the device can not be connected directly to the network, and the devices themselves were designed to be invisible to the network.
 
But that's not how people used them<G>!
 
I modified quite a few TI-101s to add a holding coil and a simple switch to the front panel. The switch places the coil across the tip and ring, and will draw dial tone, or accept an incoming call. With that in place you need only connect the TI-101 to a POTS line, or a reasonable facsimile thereof. As an added benefit, there is no transmit audio, so you don't need to disconnect the handset transmitter.
There were also a number of companies selling modified "type 500" subsets with "off-hook" and "xmit kill" switches built into the housing. The first switch simply sat in parallel with the hookswitch, and the second interrupted the transmit side of the handset. There might have been something else we had to do, but I do not remember the details any longer - you might check some of the broadcast engineering web sites for ideas if you want to try that.


Holler if you have questions...

Thanks Bill, the mod you mentioned sounds interesting.As I understand it taking the handset off the hook makes a connection. From looking at the schematic, the signal simply passes through the primary of a transformer to ground. The signal then goes on to the secondary. ...I might have questions.Thanks for the offer!

Update- The setup seems to work fine without the unhook mod if I simply use my existing phone as the unhook device. I then need to make sure that phone isn't near the rest of my gear which isn't really a problem . I monitored through my  recorder. This way I can hear the actual signal as recorded. 
 
The TI-101 seems a bit noisy in monitoring, but the noise doesn't appear to be recorded to the degree I hear it when recording. Actually you don't hear the noise on the recording. I don't think this box was in a smoke free studio. The pots need lubed but I don't think this is going to solve the noise issue.Might have a leaky cap...or were these boxes that noisy? Seemed a bit excessive to me, even for older electronics.

Indeed, my little mod was to let me work without a telephone connected, it is not necessary at all.
 
As far as noise goes, I never had noise problems with the TI-101s - and I probably used at least half a dozen over the years, not including the one I have now. I'd do a little digging, the most likely candidate is the power supply. Well, that's assuming that you don't have any interface issues... and a quiet telephone line<G>!

Almost off topic. My Dad worked for BC Telephone for 40 years, (now known as Telus,) he explained that party line ringers where all 90 Volts, but each phone had a capacitor ( dumbed down explanation) that filtered out the cycles. Party one was 20 cycles, party 2 way 40 cycles etc.. I was just a kid, so memory might be off,,,well it's always off,, but... 
He was responsible for big office building networks in Vancouver,,, he retired when they started talking digital... he worked with 100's of pairs of colour coded wires.. 100% analog. His first job was a telegraph operator. 
 
http://www.cowboycountrymagazine.com/2011/03/fence-phones/
 
 

check on Amazon.... a quick search turned up a number of things