[Gryphone]

Originally Posted by andy

I'm wondering if this is why there is some difficulty reaching it from some providers, who may have a dialplan that does not allow for this

I haven't heard of any troubles reaching these 13-digit numbers yet, however the +49-157 prefix is pretty new, when it was introduced there was some trouble reaching it with some providers, these problems should have been solved by now though.

Do other e-plus and their mnvo SIMs have these longer numbers, or is this something gradually being introduced across all German networks?

E-Plus was the last operator to introduce 13-digit numbers, T-Mobile and D2 already used them since 2001, O2 has had them since 2003.
However you can still get a 12-digit number, especially when you sign up for a contract you'll probably get one and even with most prepaid cards there's a chance of getting one.