Only for roaming customers though. Here in Europe a customer in their home country can only use their home network unless they're calling emergency services. Only when roaming multiple options would be available.
So I wouldn't expect all that much extra load really.
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45 minutes later, another reply here (https://news.ycombinator.com/item?id=44674209) suggests that Vodafone and O2 are indeed experiencing issues.
When this happens in Canada (usually a Rogers outage), the other networks seem to have a hard time dealing with the extra load.
Only for roaming customers though. Here in Europe a customer in their home country can only use their home network unless they're calling emergency services. Only when roaming multiple options would be available.
So I wouldn't expect all that much extra load really.