If they
RING YOUR PHONE (which would tend to imply they're using a phone, not a modem), then there is absolutely no way they can get any information from your computer, or even work out if you have a computer on the end of the phone-line!
Also, assuming they were using a computer and not a phone, see the below project-statistics for an indication of how difficult it is to crack 128-bit encryption (the below isn't even for 128-bit, it's for 64-bit - see the bits in bold for the important info):
RC5-64 encryption test:
Total Blocks to Search: 68,719,476,736
Total Blocks Tested: 56,878,907,073
Overall Rate: 381 Blocks/sec
Total Keys to Search: 18,446,744,073,709,552,000
Total Keys Tested: 15,268,315,356,922,380,000
Overall Rate: 102,385,059,633 Keys/sec
Percent Complete: 82.770%
Time Working: 1,726 days
There have been 328,140 participants
since the beginning of this project.
15,578 of them were active yesterday
and of those, 3 were brand-new participants.
That's a lot of computers working together to try to get through a 64-bit encryption key...