Turkey

Turkey doesn't allow foreign handsets to be used on their network for longer than a "short period". This is advertised as being 30 days, but can be as short as 18. You can register your handset with the government to get access, but it takes paperwork and time and money. You can also buy a cheap Turkish phone, but again you need to provide paperwork showing who you are. I have never taken the same phone to Turkey for more than two weeks, so cannot say if this time period is reset for each visit.

That said, you can still buy PAYG sims if you are planning on being in the country for only a short period. The tarrifs were once horrendously complicated, but are starting to be a lot simpler.

There are certain coastal areas where a Greek island is close enough for you to use their mobile networks (and thus also gain from the cheaper intra-Europe rates). However, these are rather limited.