If I use tel: I should write the international phone code, like that.
<a href="tel:+6494461709">61709</a>
So far, so good, but I can't find information on how to write a cell phone number in an "international" way, if there is one.
When dialing a number within the country you are in, you still need to dial the national trunk number before the rest of the number. For example, in Australia one would dial:
0 - trunk prefix
2 - Area code for New South Wales
6555 - STD code for a specific telephone exchange
1234 - Telephone Exchange specific extension.
For a mobile phone this becomes
0 - trunk prefix
4 - Area code for a mobile telephone
1234 5678 - Mobile telephone number
Now, when I want to dial via the international trunk, you need to drop the trunk prefix and replace it with the international dialing prefix
+ - Short hand for the country trunk number
61 - Country code for Australia
4 - Area code for a mobile telephone
1234 5678 - Mobile telephone number
This is why you often find that the first digit of a telephone number is dropped when dialing internationally, even when using international prefixing to dial within the same country.
So as per the trunk prefix for Germany drop the 0 and add the +49 for Germany's international calling code (for example) giving:
<a href="tel:+496170961709" class="Blondie">
Call me, call me any, anytime
<strong>Call me (call me) I'll arrive</strong>
When you're ready we can share the wine!
</a>