VodafoneUKのプリペイドsimカード

買ったSIMのみ（購入代金で全て収まる）

そもそもが、プリペイドSIMなわけでして・・・・。

Vodafone international roaming FAQ
Vodafone's roaming policy depends on whether you sign up for one of its Basics, Essentials, Red Extra or Red Entertainment plans.

Here, we'll outline what you get with each type of plan and in which countries you'll be able to use your monthly allowance of calls, texts and data for no extra cost.

Travelling abroad
How much does it cost to use my device abroad?

Switzerland is in our Europe Zone

There's no additional charge to use your data, minutes or texts in any of the 48 destinations in our Pay as you go Europe Zone [PDF: 677KB]. If you have a Big Value Bundle you can use your inclusive data, minutes and texts at no additional charge. Extras and Freebies can also be used in our Europe Zone. If you don't have a Big Value Bundle, Freebie or Extra you'll simply pay the same for data, calls and texts as you do in the UK

Other benefits while in a Europe Zone destination:

You can use your UK allowance or UK rates to call or text standard landlines and mobiles in other Europe Zone destinations as well as the UK
If you go over your inclusive allowance, you'll pay the same rates as you would in the UK
Get 4G speeds and 25GB of inclusive roaming data a month, on all our Unlimited plans.

スイスはこの会社がないので、Swisscomです。

Worldwide roaming partners

当然、抜ける抜けると言いつつ残留しているイタリアはEUなので、
Italy (inc. Vatican City) is in our Europe Zone

There's no additional charge to use your data, minutes or texts in any of the 48 destinations in our Pay as you go Europe Zone [PDF: 677KB]. If you have a Big Value Bundle you can use your inclusive data, minutes and texts at no additional charge. Extras and Freebies can also be used in our Europe Zone. If you don't have a Big Value Bundle, Freebie or Extra you'll simply pay the same for data, calls and texts as you do in the UK

Other benefits while in a Europe Zone destination:

You can use your UK allowance or UK rates to call or text standard landlines and mobiles in other Europe Zone destinations as well as the UK
If you go over your inclusive allowance, you'll pay the same rates as you would in the UK
Get 4G speeds and 25GB of inclusive roaming data a month, on all our Unlimited plans.

イタリアはTIMを使うこともあるようです。

一方、EU各国で売られている（土着SIMと呼んでいます）ものは、VodafoneとおフランスのORANGE以外は、
Be aware that Switzerland 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 Switzerland and can be much higher.
European providers treat Switzerland very differently: a few charge the same as within the EU zone, some limit usage in Switzerland, but many add a substantial surcharge on consumptions while roaming in Switzerland. Better check your EU/EEA provider first before use, or buy a local SIM instead.

なので、上記の国だけなら、EUローミングではなく、無国籍ローミングSIMが良いのです。

例えば、
EU roaming
The default rate is CHF 4.50 for 10 MB. You can activate for the EU, Andorra, and Monaco these data packages valid for 30 days:
200 MB: CHF 4.90
1 GB: CHF 14.90
3 GB: CHF 39.90
がSWISSCOMの価格。

あとは、LTEの周波帯でしょう。
SWISSCOMは 800, 1800と2600 MHzがメインで2100も予備で使われているようです。

イタリアでは、 : 800 MHz, 1800 MHzと2600 MHzなのですが、Vodafoneは大都市一部でバンド1（2100）を使っているようです。イタリアは期待せずにいれば、そこそこ快適に使えます。