Norma,
In order to answer your question, further information is needed. Which US cell network are you using? If it's a GSM phone, is it quad-band (ie: has the two European frequency bands)? Are you going to be roaming with your home network or using a local SIM when in Europe?
Assuming you'll be roaming with a quad-band GSM phone from either AT&T or T-Mobile, the "numerically challenged" individual will only need to dial your number as usual (as if you were at home) and the network will find you. If that's the case, you might want to use the voice calls sparingly, as the cost for incoming calls is a bit pricey.
Also, as someone else suggested make sure they cconsider the time difference before calling. I've also been awakened in the wee hours by an inconsiderate moron that didn't bother to check the time.
Cheers!