One of the big disincentives to use your mobile phone overseas is the roaming charge double-whammy: not only does the caller pay to ring you, but you get slugged with a charge as well. Roaming SIM provider TravelSIM has eased that pain a little, offering free receipt of calls in 117 countries.
You can see which countries offer no charges for inbound calls in the list linked below. If your callers don't want to pay high rates to ring your TravelSIM numnber, they can also optionally contact you using TravelSIM's 1800 number, which then gets charged to your receiving account at a flat rate of $0.21. TravelSIM already offers cheaper data services in a bunch of European countries, so it might be worth considering, especially if you travel regulalry.