Your son will have to pay in both cases and it is expensive. The best thing to do is get a prepaid card in the US and maybe use WhatsApp to make calls to you in Germany.

Good evening,

Thank you so much for your quick reply.

i do not understand why he should pay : he has a German number and he calls a German number ? 
could you just explain ? 
for the other question, I understand that it’s better to have a US prepaid.

Hello ​@Odile,
thank you for contacting the community with your question. We have summarised all the information on the topic of usage abroad in this article Roaming for contract customers. If you have any further questions, please get in touch with us again.
Best regards Bianca

Hello,

many thanks for your answers. Many things are still not so clear but I will find more .

have a nice day,

You can ask specific questions here if the information is not clear.

Calls between two German numbers are still international calls if the caller is not in Germany as the SIM card is using a foreign network provider. Roaming means the ability to use networks in other countries, but not that this is possible free of charge. For the same reason, if you call a German number that is currently roaming in the US, the person you are calling will pay for that call.

Within the EU there are no costs because of the EU roaming directive.

​@bs0 not exactly… The caller pays the fee to the German border (as the call receiver is in germany).

From Germany to the other country will be payed by the call receiver…

​@schluej That's no different to what I wrote ;-)

​@Odile Google for maaltalk and for a plan US. Have plans for US phone and data. Calculate for US / Canada about 50-70 US Dollars. And you son should turn off his o2 simcard travel outside EU. If you want reach use WhatsApp or Facebook Messenger calling.