中欧旅行のSIMカードについて

EUではないので

国内調達可能なのは
www.ais.co.th
SIM2Fly .. Anywhere
Supported Destination
Global
SIM2Fly 899 Baht
2. Albania NEW!

8. Belgium
9. Brunei
10. Bulgaria

28. Hungary

35. Japan
36. Kuwait
37. Laos

42. Macedonia
43. Malaysia
44. Malta NEW!
45. Mexico
46. Myanmar

62. Serbia

www.three.co.uk
Go Roam destinations
Unfortunately Albania is not a Go Roam destination yet. We're adding new destinations all the time though, so watch this space!
Unfortunately Kosovo is not a Go Roam destination yet. We're adding new destinations all the time though, so watch this space!
Unfortunately Macedonia is not a Go Roam destination yet. We're
adding new destinations all the time though, so watch this space!
Unfortunately Serbia is not a Go Roam destination yet. We're adding
new destinations all the time though, so watch this space!
Hungary is a Go Roam in Europe destination, so you’ll be able to use
your data (and call and text back to the UK) without any extra charges.

最新のiPhoneがあれば、
www.ubigi.com
PLANS & COVERAGE
EUROPE Extended Zone
NORDIC & BALTIC 3GB valid 30 days US $19
Continental data plan: (See full country list below)
Albany (Albtelecom)
Bosnia and Herzegovina (HT Eronet)
Hungary (Telenor)
Macedonia (VIP Operator)
Montenegro (Telenor)
Serbia (VIPNET)

上記は電子SIMなので、物理SIMなら、
www.transatel-datasim.com
Europe Travel SIM Card
ubigiと同じ地図なので。
USD25.00 、送料入れて一週間ご受け取りで26ユーロ。

これを勧めている人は皆無ですが、金に糸目をつけないのであればありかも。
Albania
International roaming
Albanian operator Albtelecom and Kosovo's Vala have renewed an existing agreement that enables the Roam Like Home service for customers. Users visiting Albania or Kosovo are charged the same prices as in their home country. The costs of communications between the operators have been considerably reduced and the benefits extend to both voice and SMS messaging as well to internet access on 4G networks.

ハンガリー以外は、
Be aware that Albania is not part of the EU, nor the EEA, where from June 2017 new international roaming rules are enforced. So roaming rates are not capped or regulated in the country and can be much higher on most European SIM cards roaming in the country. Only on some Vodafone plans Albania is included in the Euro zone. Better buy a local SIM card instead.

これも意味がないでしょう。
Serbia
International roaming
Special discounted roaming rates apply to Montenegro, Bosnia and Herzegovina and Macedonia at 3 RSD per 100 KB.

ＫＫ外遊

Ａｍａｚｏｎなどでよく見る周遊カードを日本で買って行きました。

今、パッケージを見たところ、コソボとボスニアヘルツェゴビナ以外はカバーしているようです。
私は通信速度には拘らなかったので曖昧ですが、３Ｇになる事は多かったものの　圏外で使えなかった記憶はありません。（訪問先が異なるので参考にはならないですが・・・）
設定も、日本の通信会社のプロファイルを削除する方法だけメモして行けば、特に難しいことはありませんでした。
国境を越えた後通信が止まった場合も、一旦「モバイルデータ通信」をＯＮし直せば問題ありません。